Re: Denver and 200 mile radius in May
Some of my favorite rivers:
The classics, if you don't mind a bit of a drive, are the Frying Pan and the Roaring Fork outside of Aspen. Check with Taylor Creek Fly Shop, I've found them to be very helpful and they do not try to push guides or equipment on you. Weekends can be busy, particularly near the dam.
Closer in is the Gore through Vail(stay away), Minturn, Avon and Edwards. Generally easy water to wade and usually w/out anglers. Sorry, but I've never found a shop there worth recommending. (Cutthroat Anglers website will have Gore river reports)
Closer still is the Blue, which fishes wonderfully below the dam and through the town of Silverthorne, try Cutthroat Anglers and their website for river reports. It too can be busy on the weekends.
Good luck and let me know how you do.