Near Denver:
North Fork South Platte (NFSP)
Baily: IV/V (from what I hear, no fun at all)
Foxton: III (great at higher water 400+cfs, beautiful scenery)
South Platte
Waterton Canyon: III+/IV (short but sweet)
These runs are served by the Roberts Tunnel, which can drop a few hundered cfs into the NFSP at Grant, Colorado. Often release schedules are posted on mtn buzz.
Arkansas:
Royal Gorge: III/III+
Numbers/Fractions/Browns: When do the reservoir releases stop? Mid Aug?
Hope that helps!