In 1999-2000, Idaho hosted over 63 million travelers. Tourists come to Idaho primarily for outdoor recreation—river trips, skiing, camping, hunting, fishing, and hiking. There are 19 ski resorts, of which by far the most famous is Sun Valley, which opened in 1936. Boise is the most popular destination within the state.
Tourist attractions include two national parks, the Craters of the Moon National Monument and the Nez Perc National Historical Park, and the Hell's Canyon and Sawtooth national recreational areas. A sliver of Yellowstone National Park is in Idaho. Portions of the Lewis and Clark Trail and the Oregon Trail lie within the state as well.