I ride a mountain bike, a road bike and a road tandem but not a mountain tandem. So, my recommendations come without first hand experience.
Centennial Cone is a moderate mountain loop just west of Denver (up Clear Creek Canyon). It is about a 13 mile loop. Most of it would probably be pretty doable on a tandem. However, check the schedule as it alternates weekend days between bikes and no-bikes.
Another easy suggestion is the Highline Canal Trail. It is a hard-packed gravel road that follows an old irrigation canal. It can be quite scenic and wanders all over the Denver area. You can start & finish most anywhere along its length (and return). There is almost no elevation change along the road and you are away from cars for almost all of the trail.