Hells Gate State Park is perhaps most famous for being home to Hells Canyon, a scenic stretch of the Snake River that's one of the top tourist attractions in Idaho. While many people head to Hells Canyon to go rafting and kayaking, the Hell Gate State Park itself is a nice choice for a day-trip to enjoy the great outdoors and cool off next to the Snake River.
It costs $5 to access the state park with a car and, upon arrival, you'll find a clean and beautiful riverside park with picnic areas, a few scattered river beaches, and barbecue grills for cooking out. There are also two worthwhile museums on the State Park grounds. The first is the Lewis and Clark Discovery Center that has a half hour video outlining the famous expedition to the Pacific coast and a large outdoor area full of sculptures and informative displays. It's a great place to bring the kids to help get them interested in this important period of American history. The other museum is the Jack O'Connor Hunting Heritage and Education center. The Jack O'Connor Center gives an overview of the famous naturalist, author, and big game hunter and has a pretty impressive collection of his hunting trophies from around the world. If you're interested in hunting and wildlife, it makes for a pleasant hour or so of your trip.
There are, of course, plenty of RV and tent-camping sites in Hells Gate State Park if you're looking to explore the trails and viewpoint a bit longer. I'd suggest just making a day-trip out of the park and enjoying a morning of hiking before settling down for a barbecue and some swimming in the refreshing Snake River in the afternoon.