Visit the U.S. National Park Service page: “Find a Park”
Then, either roam through the park list or choose to limit the search by Park, State, Activity or Topic.
Activities include Dog Sledding, Flying, Museum Exhibits, Surfing and Wildlife Watching. Topics include Ancient Seas, Caves, Caverns & Karst, Forts, Volcanoes and Wetlands.
Another option is the National Recreation Trails. There are three designated types: National Scenic Trails, National Historic Trails and National Recreation Trails.
Happy Wandering!