One of the best experiences was "tubing" down the Yampa Valley River.  Basically you rent a large inner tube, put on funny rubber shoes for traction and then you will somehow manipulate yourself bottom first into the middle of the tube and you're off on a three mile adventure down the beautiful Yampa Valley River.  There are several companies that rent tubes.  Just ask around when you are downtown and you can find out where they are.  Remember to stand up when it's time to get out, otherwise you will continue to float down the river, lol.  Also it's easier going down the river alone because there are large rocks with some just under the surface that you must manipulate around so it's better not to be tied to someone else.    The cost was $17.00 per person and that gives you the tube, rubber shoes and life jacket plus a shuttle that takes you back to your car.  You can't take things with you on the tubes so you either leave your things there for the staff to secure or you lock them in your car.  The area is pretty safe so that is not a problem. Happy tubing!