Awesome stuff in Colorado that I have done and recommend:
Whitewater rafing: The Arkansas river will be the only one running decently in August (maybe the Colorado River in Glenwood Springs but that isn't as fun). Starting base in Buena Vista or Salida.
Trains: Scenic train rides in Georgetown (gold mining history) and Canon City (through the Royal Gorge, deepest gorge in the state, site of the highest suspension bridge in the US)
Camping: it does get cold at elevation in the mountains but, its free/really cheap just about everywhere. My current favorite free camping site is in East Vail, take the little gravel pullout under the i-70 overpasses right by the Gore Creek Campground.
Festivals:
http://www.coloradoinfo.com/festivals-fairs#August Jazz, whitewater, cycling, beer, bluegrass, etc.
Garden of the Gods: State Park in Colorado Springs where giant rock formations formed by the tectonic plates squeezing together.
Pikes Peak: Same as Mt Evans, drive to the top of a 14,000 ft mountain on a crazy switchback road (or take the cog train)
Beer: All along the northern front range (Denver, Boulder, Longmont, Lyons, Ft Collins), there are awesome breweries. I love taking a walking tour of the downtown Denver breweries (especially because I can take the light rail there and don't need a DD).
http://www.denvermicrobrewtour.com/Hike: This one is free and EVERYWHERE!!! Red Rocks park has some great trails, or drive up a little higher to Mt Falcon State Park or the new Staunton State Park. Rocky Mountain National Park also has a ton of them.
Hot Springs: 2 types, free and resorts. The free ones you hike to a place where people piled rocks to collect water in pools. The resort ones, you pay an entry fee and they have multiple swimming pools/hot tubs with water piped in. Mount Princeton Hot Springs outside Buena Vista is amazing. Long list of them at
http://www.trails.com/toptrails.aspx?area=10541 Let me know if you want any other ideas.