Assuming you are a rafter of III - IV ability these would be worth considering:
Lochsa, Alpine Canyon of the Snake (maybe other sections too?), Payette river system, Cataract Canyon of the Colorado, Gunnison Gorge, Arkansas River System, American river system in Cali.
Plenty more but this is a good place to start. With time you can hit a main section and explore nearby rivers/creeks. Have fun!