Henry's Lake is a beautiful state park located in Eastern Idaho not far from Yellowstone National Park. It is a nice place to take a break from the crowds and hectic pace of Yellowstone. The park is named after an earlt explorer in the area Major Andrew Henry and is open (weather permitting) from the Thurday before Memorial Day until mid-October.
Henry's Lake has some nice, scenic hiking trails. They are all relatively short and low in difficulty.
Henry's Lake has 44 campsites, plus a few cabins available if you wish to stay at the park.