Don't settle for just a walk in the national park. Protected spaces across the country (and in some oceans) offer adventures that go beyond hiking and camping. Be prepared, though: Some of these activities require extra padding.
Caving: Get your thrills in underground nooks and crannies on a cave tour. New Mexico's Carlsbad Caverns, for example, offers self-guided tours on select routes, as well as ranger-led outings to such dark and rocky places as Slaughter Canyon Cave. Fees from $6. Information: (505) 785-2232 or nps.gov/cave.
Other parks with caving include Jewel Cave National Monument (South Dakota), Mammoth Cave National Park (Kentucky), Timpanogos Cave National Monument (Utah), Oregon Caves National Monument (Oregon) and Sequoia and Kings Canyon national parks (central California).
Mule rides: Most visitors to the Grand Canyon have seen mules carrying passengers down the slope, if they themselves were not said cargo. You can also mosey along on these beasts of burden at Bryce Canyon in Utah and Kalaupapa National Historical Park in Hawaii. At Bryce, for example, wranglers lead mules and their riders to the canyon floor, passing through hoodoos and other artful rock formations. Information: (435) 679-8665, www.canyonrides.com; from $40.
Sledding: Sledding is always in season at White Sands National Monument in New Mexico. The park's tall white dunes are ideal for bombing down — and no ice scrapes. (Instead of snow, visitors slide down undulating expanses of soft gypsum.) The gift shop sells waxed plastic snow saucers. Information: (505) 679-2599, nps.gov/whsa.
Diving/snorkeling: Nearly half of Channel Islands National Park in Southern California is underwater, so the best way to tour the five islands is by snorkel or scuba. Join the fishes off Santa Barbara, Anacapa and Santa Cruz islands; sights include kelp forests, caves, coves, seals, spiny sea urchins, whales, dolphins and more. Contact local dive companies for more information. (The park also has good waves for surfing; try Santa Cruz, Santa Rosa and San Miguel islands.) Information: (805) 658-5730, nps.gov/chis.
Additional parks with underwater activities include Isle Royale, where divers can swim among shipwrecks in the northwest corner of Lake Superior; Dry Tortugas National Park off Key West; the National Park of American Samoa, which is rich with coral reefs; and Virgin Islands National Park on St. John.
Scenic flights: For an eagle's-eye view of Kenai Fjords and its glaciers, ice fields and brown bears, grab a window seat in a small plane or helicopter for a tour of the vast Alaskan park. Contact the Seward Chamber of Commerce at (907) 224-8051 or sewardak.org for tour operators.
Other parks with flightseeing include the Grand Canyon; Acadia in Maine (biplane and glider rides also offered); and Utah's Bryce Canyon, Canyonlands and Capitol Reef.
BioBlitz: The National Park Service is running the high-adrenaline research race, a 24-hour expedition in which teams of scientists, families and adventurers scour a park, looking for as many plant and animal species as possible. The next will be at the Big Thicket National Preserve in Texas (Friday and Saturday), Acadia in Maine (Aug. 8-11) and the Boston Harbor Islands National Recreation Area (Aug. 18). Free, but registration is required. Check the schedule of events on the Web site for each park. Start at nps.gov and search for park names.