There are some lovely things to do and see in Idaho during the summer months as well. They have some really good whitewater rivers and a well established tourism industry surrounding that which caters to all levels.
There's also some good fishing lakes and other things like horse riding and golf.
But the natural attractions are what really do it for me. There are a bunch of national parks and things to see like the balancing rock, or the enormous forests and canyons.
There's also an area near some dunes where they've set up an observatory so you can see the stars without any light pollution. I've heard that's really worth having a look.