The Arkansas would be another good possibility.
Someone correct me if I'm wrong, but I think the only runs in Colorado, for which you must apply for a permit in advance, are the Yampa and Green rivers through Dinosaur National Monument. Everywhere else, you can just show up with your boat and the proper equipment. You may have to self register for some, but none on the Front Range.
have fun on the river!
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse