Idaho, Montana, and Wyoming
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.2. Yosemite National Park
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.3. Grand Teton National Park
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.4. Great Smoky Mountains National Park
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.