Definitely visit the Frank Church Wilderness/River of No Return. Craters of the Moon Natl Monument and Hell's Half Acre are really cool if you like semi-arid volcanic stuff. Boise, Salmon-Challis, Payette, and Targhee National Forests are all pretty sweet to hike/camp in. Salmon-Challis has a lot of fires right now though, could be shut down. Tons of rivers all around Idaho, most of any state I think, so definitely take some time to do kayak/canoe/fishing. Sawtooth and Centennial Mountains are pretty neat but very touristy especially if you're out by Stanley/Ketchum, very affluent area also. Can't speak for Northern Idaho much but I hear Coeur d'Alene and the surrounding areas are nice.
