Or a "southern Colorado" strategy: From COS going west, Sand Dunes Nat'l Monument, Pagosa Springs, Durango, Cortez/Mesa Verde and maybe a return trip by Salida. COS to Durango is quite doable in a day and then hit Mesa Verde fairly early the next morning, before the mid-day heat really hits. Even if not your initial choice, you may want to keep it in your back pocket in case of expected bad weather in the Denver-Estes-Winter Park area of the weekend you will be here. Weather does change rapidly, so I wouldn't change plans on the chance of a thundershower, but different parts of the state can have radically different weather.

Also, I am a great fan of RMNP, but there are many places driving through the park where it is impossible to pass slow moving vehicles that are slowing down (but not pulling off) at every new vista. Assume Estes to Winter Park/Grandby could take a few hours, despite the mileage.

Possibly useful website: http://www.coloradolinks.net/Colorado_Festivals.htm