The South Fork of the Snake River in eastern Idaho is world famous for its dry fly fishing for cutthroat trout. There's limited bank access, but it's an awesome river to float in a driftboat, raft or cataraft. It's also a good place to catch trophy brown trout.
Another option: Horsethief Reservoir is a great mountain getaway. It has great scenery, lots of camping and fun trout fishing for bank anglers and boat anglers alike. It's also a good place to escape the Valley's sweltering heat.