Blow Their Minds: With more than 500 geysers, almost four dozen waterfalls, plus hissing steam vents and exploding mud pots, kids will see something exciting everywhere they turn. "My kids still talk about the bear that walked in front of our car and the herd of bison that was so close to us that we could have rolled down the window and touched them," says Kate Rothstein, a mom of four in Pittsburgh. The Hayden Valley, in the center of the park, and the Lamar Valley, in the northeast corner, are among the best places for drive-by spotting of some of the park's 400-plus animal species, which include an estimated 30,000 elk. On foot, follow the easy Garnet Hill Loop and Trout Lake Trails (both in the northeast section) as well as the Storm Point Trail (near Yellowstone Lake) for the greatest chance to see wildlife.
Hang With Rangers: Take part in the Wildlife Olympics, held three times a week in various locations. "We set up seven stations that help children and parents compare their skills to those of the animals at the park," says Bob Fuhrmann, manager of the youth and volunteer program.
Plan The Trip: The closest airport is in Belgrade, Montana, but fly to Jackson Hole, Wyoming (two and a half hours away, near Grand Teton, another park you'll want to visit on the same trip) or even Salt Lake City (a five-hour drive). "We found it was easier to use our airline reward miles through Salt Lake City, because it's a hub for major carriers," says Leigh Sliwinski, of Canton, Ohio, who visited last summer when her daughters were 5 and 11. Try to spend at least a few nights inside the park because it feels different in the early morning and, besides, you'll maximize your time, says Ford Cochran, director of programming for National Geographic Expeditions and a dad of two. "Our favorite place to stay is the Old Faithful Inn," he says. The inn is open May 8 to October 11 -- beat the crowds by going as early or as late into the school break as you can. Rates start at $199 per night for a West Wing room; yellowstonenationalparklodges.com.
Blow Their Minds: From the south entrance, head to the Mariposa Grove to see 500 mature giant sequoias, among the world's tallest and oldest trees. While kids will be able to spot some from the parking lot, they'll really be wowed during an easy 3/4-mile hike to check out the Grizzly Giant (the largest tree in the grove) and the California Tunnel Tree (they can walk through it -- it was cut as a passage for stagecoaches in the late 1800s). After this month, the entire grove will have limited access for two years for renovations; check the park's website for updates.
Hang With Rangers: Take a Junior Ranger walk starting at the Happy Islands Nature Center. "Just before my kids were sworn in as Junior Rangers, we saw a mama bear and her two cubs!" says Nicole Dodson-Sands, a blogger for Trekaroo.com.
Plan The Trip: Even though Fresno airport is closer, you'll find more options and lower fares by flying into Sacramento, which is about a three-hour drive away. While some of Yosemite is open year-round, you don't get the full experience unless you visit in late spring to early fall. Stay at the Yosemite Lodge at the Falls, where rooms start at $250 per night, or make a reservation for one of the park's 13 campgrounds.
Blow Their Minds: "You leave Yellowstone, the Teton's sister park, and it's like coming to another world, with soaring mountains and beautiful lakes," says Cochran. Start exploring by taking the Jenny Lake shuttle boat ($15 for ages 12 and up, $8 for ages 2 to 11) and hiking about a half mile to Hidden Falls (an 80-foot waterfall) and then, if the kids are up for it, another steeper half mile to Inspiration Point for Instagram-worthy mountain views.
Hang With Rangers: Borrow a Nature Explorer's backpack with a journal, lenses, field-guide cards, and more activities tailored for kids ages 6 to 12 from the Laurance S. Rockefeller Preserve Center. "We found animal footprints on the trail and made impressions of them using the plaster mold in the backpack. In fact, we still have them on our front porch," says Rothstein.
Plan The Trip: Many families spend four or five nights in Yellowstone and two or three in Grand Teton. "The park's tent cabins, with two log walls and two made of canvas, are great for families who want to ease into camping," says Eileen Ogintz, founder of TakingTheKids.com. Each cabin comes with four bunks (they have mattresses; you supply the bedding). Summer rates start at $62 per night. If you don't want to "rough it," consider staying at the Jackson Lake Lodge, which has its own heated swimming pool. Rates start at $215 per night; gtlc.com.
Blow Their Minds: Thanks to the colorful leaves on the 100-plus species of trees, the park looks like a rainbow when you drive through in mid-October to early November. Hop on the Little River Road near the Sugarlands Visitor Center, and take in the scenery for 25 miles to the open valley of Cades Cove; your kids may spot deer, coyotes, groundhogs, and turkeys. Follow the short Cades Cove Nature Trail to see waterfalls.
Hang With Rangers: On Tuesdays in the summer, a ranger dresses in a blacksmith costume and invites kids in groups of eight to put on an apron, goggles, and gloves to learn what a blacksmith did 100 years ago. "Kids help make a triangular dinner bell that they take home with them," says Mike Maslona, park ranger.
Plan The Trip: Consider coming midweek, especially if you're headed there in the fall. If you're not within driving distance, fly into Knoxville, Tennessee -- the closest major airport. "There's enough to do inside the park for two to three days," says Ogintz. In Gatlinburg, you can rent a one- to five-bedroom cabin with a kitchen starting at $174 per night; stonybrooklodging.com.
Blow Their Minds: Even antsy kids will be mesmerized as you drive on the Going-to-the-Sun Road, a 52-mile highway that passes by waterfalls, animals such as mountain goats and sheep, and the Jackson Glacier overlook, where they can get a good view of the seventh-largest glacier in the park. "The glaciers are disappearing fast, so share with the kids while you still can," says Cochran. Stretch little legs on the mile-long Running Eagle Falls Trail.
Hang With Rangers: Take part in the Night Explorer program, where kids will peek in telescopes, learn about the sun, and use their senses.
Plan The Trip: Don't visit until July to make sure that the Going-to-the-Sun Road is open, since it's typically snow-covered from November to late June. You'll usually get the lowest airfare by flying into Great Falls, Montana (three hours away) or Spokane, Washington (around a five-hour drive). If you land in Spokane, you could stop at Coeur d'Alene, Idaho, a gorgeous lakefront town on the way. Just outside the park is the Glacier Park Lodge, a historic hotel with a pool and mini golf. Rooms with two double beds start at about $200 per night; glacierparkinc.com.
Blow Their Minds: Red mountains, green pools, and a "weeping" rock wow the kids. From the Zion Canyon Visitor Center, it's an easy walk to see hanging gardens growing out of sandstone cliffs and a rock that has water coming out of it. Then head to the Zion Lodge to hit the trail to the Lower Emerald Pools.
Hang With Rangers: Swing by the Nature Center at 3 p.m. for the Amazing Animals adaptation program that pairs science concepts with a fun game (such as lizard tag).
Plan The Trip: If you're flying, you'll likely want to land in Las Vegas -- it's the closest major airport, about a three-hour drive. Consider staying for three nights at the Zion Lodge (rooms with two double beds start at $196 per night) and then spending another three at the North Rim of the Grand Canyon (which is about two hours away). Try to visit both in early June or late August, when ranger programs are still robust but the crowds have thinned out a bit.
Blow Their Minds: The 18-mile-wide, 277-mile-long canyon is the star here, so give the kids the view of a lifetime and go beyond the crowded trail at the park's South entrance. "We drove to Desert Point and went up in the Desert View Watchtower," says Dodson-Sands. If you have a toddler who is prone to bolting, you'll be okay in the main area on the South Rim path (it has barrier walls) and the watchtower, she says.
Hang With Rangers: Take a guided 60-minute, 1-mile fossil walk that starts at the park's Bright Angel Lodge every morning. Families can explore an exposed fossil bed along the rim, with impressions of sponges, brachiopods, and other marine creatures from more than 270 million years ago.
Plan The Trip: Fly into Las Vegas if you'd like to visit the North Rim (lodging and services are open from May 15 to October 15) and Vegas or Phoenix to see the less crowded but somewhat less majestic South Rim. Stay at the South Rim's Bright Angel Lodge; rooms start at $100 per night and cabins at $128 per night.
Blow Their Minds: At more than 1,500 feet, Cadillac Mountain is the highest point along the North Atlantic seaboard -- and it's easy to get to by car. "Standing on top of the mountain was an extreme moment for my 8-year-old," says Jennifer Emmett, editor of the National Geographic Kids National Parks Guide U.S.A. On the way down, keep your eyes peeled for wild blueberry bushes and deer.
Hang With Rangers: Check out one of the ranger programs such as Tidepool School, where kids can explore creatures found in the tide pools with the guidance of rangers.
Plan The Trip: Check out one of the ranger programs such as Tidepool School, where kids can explore creatures found in the tide pools with the guidance of rangers.
Blow Their Minds: Start at Byrd Visitor Center to head to Lewis Falls Trail, a 3-mile, wildflower-dotted trek to an 81-foot waterfall.
Hang With Rangers: At the Loft Mountain Wayside every afternoon, hear a ranger talk about the park's bears, bobcats, and bats.
Plan The Trip: Stay in early summer if you can. At that time, there are more family programs and fewer crowds -- making the visit even better. "On our last trip, we rented a farmhouse just outside the park and drove in almost every day for a week," says Emmett. "But we also like Skyland Resort in the park." Rates start at $117 per night.
Blow Their Minds: Take the kids on a fossil hunt; the park is home to one of the world's richest deposits of fossils. In fact, five years ago, a 7-year-old girl discovered a rare saber-tooth-cat fossil at the park. Start off on the Fossil Exhibit Trail, about a 1/4-mile-long boardwalk with replicas of fossils found in the park and good signage so you can tell the kids what they're looking at. (Explain that you're not allowed to remove anything.) From there, take a scenic drive through the Badlands. The Wall is a geologic feature that stretches for 80 miles and looks like it belongs on another planet.
Hang With Rangers: Stop by the Paleontology Lab in the Ben Reifel Visitor Center during the day; you can watch them work on fossils found recently in the park, ask questions, and learn how to earn a Junior Paleontologist badge.
Plan The Trip: High temperatures average in the mid-90s in July and August so try to visit in May, June, or September instead. Last year, the park opened eco-friendly cabins at the Cedar Pass Lodge; they include a mini fridge and a microwave. (Summer rates for cabins start at $157 per night; cedarpasslodge.com.) Extend your family's vacation by visiting Mount Rushmore (about 90 minutes away, passing through Rapid City) and the Black Hills National Forest (another hour or so from Mount Rushmore).
Most of the country's national parks offer Junior Ranger programs, usually for kids ages 3 and up. To receive a badge or patch and be sworn in as a ranger, kids have to complete a booklet. You can download them from most of the park websites so you have an idea in advance of what's required. While you're on the Web, also check out the schedule of ranger programs that's usually posted for every season. Some larger parks, such as Yellowstone and Yosemite, offer more than 100 options weekly.