Upper Colorado River, in Grand County. Contact Red Tail Rafting at 970.726.1197 and tell Tony or Jeff that Paul from King Mountain Ranch sent you, and says hello from Montana. Best family river stretch with the best company running on it.
Nearby you will find excellent mountain biking for the whole family at Bike Granby Ranch (formerly Sol Vista Basin), family horseback riding catered to your needs and abilities with Samuelson Outfitters (call Kathy at 970.726.8221, tell her I sent you), and the highly scenic and tourist friendly environment of Rocky Mountain National Park and the town of Grand Lake. You could also talk to Spirit Lake Polaris (Matt Woody, 970.627.928
for ATV rentals on the Stillwater Pass trail system, don't know their age limits for backseat riders. Lots of easy (<1 mi.) hikes, including a few to beautiful, high mountain lakes (Lost Lake off the Stillwater trail system) could round out your trip.
Recreate in confidence knowing that if anything happens, the members of Grand County Search and Rescue will be there to help. Not that they'll have to, because the companies I recommended are top notch.