In Colorado, ski resorts are turning themselves into terrific places to visit during summer months -- check out the photo above, from Winter Park.
Also in Colorado is a unique opportunity for families: a stay at the YMCA of the Rockies. (Book early!) YMCA of the Rockies has two centers: Snow Mountain Ranch (5100 acres) and Estes Park Center (860 acres). Guests stay in lodges or cabins and enjoy a bonanza of outdoors activities. Horseriding, archery, zip line, High Challenge Course,... Affordable pricing, too (as is typical of "family camps".)
Straddling Wyoming and Montana, Yellowstone National Park is an amazing destination. Expect crowds in summer-time; try to find "the roads less traveled".
White water rafting is excellent fun in summer: check out a family-fun company with rafting trips in Utah, Colorado and Idaho. "Ranch and Raft" is an interesting combo trip. And speaking of ranches... Dude ranches in Colorado stay open in summer months.
- See these and more Rocky Mountain Vacation Ideas for Families