The South Fork of the Boise River is usually at a prime level for floating, and the stonefly hatch usually comes off in June. It's a great time to float the river and cast dry flies to the bank for rainbows. Expect big fish, but not big numbers of them.
Another option: Silver Creek's brown drake hatch usually happens in early June. It's the easiest time to fool the creek's notoriously finicky trout.