IF you are flying into Denver you should rent a car. In the summer the mountains are cool so you should be open to explore. I know in the winter for skiiers they have shuttle services between the ski towns in the Vail/breckonride area. I dont know if they operate in the summer.

My own personal bias...If you stay in Vail is drive to Maroon Bells and Aspen then over independence pass then to beauna vista then back to vail or up route 9 to breckonridge.

I would suggest hppoing between vail and breakonridge say staying 2 nights in each