K2 - Welcome to Colorado!
Lots of good info above. I recently boated with some newcomers to Colo and it seems there's a misperception elsewhere that you have to have a permit for almost any river in Colorado.
If you aren't looking for whitewater, during runoff season there's also the White River to the Green River (takeout at Sand Wash), and the Yampa River above Deerlodge.
Then there's the Colorado River all the way from Pumphouse to Westwater Ranger Station (not as one continuous float), and then from Cisco all the way to Potash just below Moab.
If you float below Potash on the Colorado or Mineral Bottom on the Green, you'll need to take a jetboat back up from the confluence to Moab or be ready to run Cataract Canyon and have a plan to get to Hite if Lake Powell's not really low. ...Or row really hard back upstream.