Shoshone Falls is a stunning waterfall on the Snake River near the town of Twin Falls, Idaho. The falls are over 200ft. high (higher than Niagara, mind you) and are really one of the great natural wonders in the west. If you can, try to visit Shoshone Falls in the late spring when all the melting snow guarantees the falls will be roaring with water. Once you get to the park, it costs $3 to enter (really, it's a parking fee) and then you have a couple of observation areas, a snack bar, and a grassy area where you can lay out or go for a swim in the lake. I'd suggest taking one of the trails that leave from the parking lot. Each trail doesn't take more than an hour and a half at most and they pass by several view points which are arguably better than the main one at the entrance.
Even the drive to reach the falls is worth the journey. The road twists and winds through the Snake River Canyon and the views are mesmerizing. Try to fight the urge to pull over every 30 seconds and snap a photo! This is what I'd suggest: pack a picnic lunch (cheaper and better than the restaurant) and head to the falls. Take a walk around the visitors center and then settle down for a nice lunch on the grass. Afterwards, you can walk it off on one of the trails and check out the birds and snap some photos of the falls. You won't regret it....it's simple beyond words.