Craters of the Moon is awesome. Bring lots of lights for exploring the lava tubes, and maybe some knee pads. Hike to the top of one of the cones for a sunrise or sunset. My favourite memory of the place was watching the Perseid meteor shower, you will be about a week late for it unfortunately. Watching those come down while in the park was as close as I'll ever get to being on another planet. As a 15 year old it was really cool. But if you are somewhere else with a big dark sky August 11, 12 look up!
http://earthsky.org/astronomy-essent...guide#perseids
Hit up Lewis and Clark cave in Montana if you get a chance.