With a population of about 2,200 grizzlies, Katmai National Park in Alaska offers some of the best bear-watching anywhere. The Brooks Camp area has three viewing platforms where visitors can safely watch bears fish for salmon along the Brooks River. Peak viewing times are July and September.
Fireflies in Great Smoky Mountains National Park in North Carolina and Tennessee put on a dazzling mating-display light show for about two weeks in late May to mid-June (viewing dates are announced in late April). While viewing is free, visitors must enter a lottery to get a parking pass, and the passes and shuttle buses to the viewing area require a nominal fee.
Float down Cedar Creek on a leisurely four-hour canoe tour with rangers in South Carolina's Congaree National Park. Watch for bald eagles and enjoy the dense old-growth bottomland forest, the last of its kind in the country. Reservations must be made in advance online, and groups are limited to 12 people. Children must be at least 6 years old to participate.
Ranger-led paleontology tours offer close-up views of fossilized animal tracks in a part of California's Death Valley National Park that is typically off-limits. The 7-mile round-trip hike is strenuous, and no kids under 10 are allowed. Hikes are limited to 15 people, with spots filled via a lottery.
Not afraid to get wet? Go off trail on a guided hike through some of Everglades National Park's famous sloughs, perhaps better known as swamps. Participants must wear long pants and sturdy closed-toe shoes to participate. Groups are limited to 15 people 12 and older; reservations are required.
A short hike leads to a hot spring that bubbles up near the Rio Grande in Texas' Big Bend National Park. Early settlers thought the dissolved mineral salts in the water had healing powers. Visitors can test the theory by taking a soak in the 105-degree pool in the foundation of an old bathhouse.
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Visitors should keep an eye out for urchins, barnacles, sea stars, anemones, and scores of other creatures in the vibrant tide pools of the Channel Islands, off the coast of California. The park recommends at least six sites for tide pooling, and visitors can learn more during ranger-led talks at the Ventura visitor center.
Stargazers flock to Grand Canyon National Park in Arizona every June for a weeklong star party with nightly slide shows, telescope viewing, and ranger-led constellation tours. No registration is required, and there are observation sites on the north and south rim of the canyon.
Visitors who want to explore Yellowstone's backcountry with an expert can participate in a guided five-hour Ranger Adventure Hike to such places as the Yellowstone River, Avalanche Peak, Garnet Hill, and Lone Star Geyser. Groups are limited to 15 or 20 hikers and reservations are required.
Though not for the faint of heart, a ranger-led hike in Kenai Fjords National Park in Alaska offers unparalleled views of the 700-square-mile Harding Icefield. This eight-hour trip includes substantial elevation gains and is designed for more experienced hikers. It's offered Saturdays in July and August.
Bored with day hikes? During full moons, rangers lead visitors on 1- to 2-mile nighttime hikes in Bryce Canyon National Park in Utah, offering a spooky perspective on the rugged landscape. The hikes are free, but tickets are distributed via lottery during more popular months. Hiking boots or shoes are required, and kids under 6 are not allowed.
During fall, wildlife such as bears, elk, and bison are on the move in Wyoming's Grand Teton National Park. Bring binoculars and team up with a ranger for a three-hour wildlife caravan offered daily in September. Each caravan is limited to 10 vehicles, so reservations are required.
True to its name, Glacier National Park in Montana can be a formidable place in the winter, with many activities off-limits. But rangers lead two-hour snowshoe walks on winter weekends, perfect for visitors who want to appreciate the park while it's quiet and uncrowded. Participants can bring their own snowshoes or rent.
From April through November, visitors to Ohio's Cuyahoga Valley National Park can participate in "quests" following rhyming clues that help tell the natural and cultural history of the area -- no GPS required. At the end of each quest, participants find a hidden box with a completion stamp and a log to sign. In all, there are 40 quests along the Ohio and Erie Canalway.
Shutterbugs can get a better feel for landscape photography during camera walks through Yosemite National Park in California. Led by staff photographers with the Ansel Adams Gallery, walks are offered four days a week, limited to 15 participants. Topics may include camera basics and Adams' famed work in the park.
Visitors to Hawaii Volcanoes National Park can take part in free cultural activities such as ʻIke Hana Noʻeau, which focuses on traditional Hawaiian handcrafts and invites visitors to take part, and Nā Leo Manu, or "Heavenly Voices," an evening concert series that introduces participants to Hawaiian music and dance, including hula.
Half-hour demonstrations by Denali National Park rangers give visitors unique insight into how teams of sled dogs work together to traverse the frigid Alaskan wilderness. Tours of the park's kennels are also available. Demonstrations are offered May through September, with three daily in June, July, and August.
If water levels allow, rangers lead kayak tours under towering trees on the scenic Smith River in Redwoods National Park, in northern California. The two-hour floats are offered twice daily on Fridays and Saturdays during the summer. Paddling gear is provided. In-person sign-ups are required, and kids under 10 aren't allowed.