4-5 days in Colorado
 

I am flying into Denver on Aug 21, planning to spend two nights there as i've never visited the city. then for the next 4 days i want to venture out for rafting, hiking and biking, and just gorgeous scenery. Willing to drive up 2-2.5 hours outside of Denver. Does anyone have any itineraries to recommend? i'm wondering if it's better to go toward theBoulder area or towards Colorado Springs area? or any other suggestions?

Neither Boulder nor Colorado Springs would be my suggestion, as it sounds like you're more into a nature trip...I would think you might find more of what you're looking for in places like Estes Park, Steamboat Springs (although that's pushing it at 3 hrs), or Vail.

http://www.estes-park.com/
http://www.steamboat.com/
http://vail.snow.com/summerhome.asp

I live in Boulder, so here's my advice:
- gorgeous scenery, driving: Trail Ridge Road through Rocky Mountain National Park - drop dead gorgeous. (beanweb24 was close with the Estes Park suggestion).
- rafting: a few places along I70 west of Denver.
- hiking/biking: Boulder is a good option, most likely less expensive than Vail. I would suggest Summit County - Breckenridge/Keystone/Dillon for better value than Vail. It's about 2 hours from DIA. You could even make a loop - Trail Ridge southwest to CO 9 dropping into Silverthorne.

You are overlapping with the Dem Nat Convention. Reservations might be tough.

The dates are the 25 to the 28th. But a lot activity takes place the week before. Keep that in mind for your planning.

I have to agree, Steamboat Springs is quite a neat little town and you can catch whitewater rafting and/or fishing charters out of there. Bucking Rainbow is the name of my choice rafting excursion company. You are going at a good time to catch some mid-level raft time.

There is also "river toobing" (floating) the Yampa river, a gorgeous paved core trail to bike/run/stroll along the Yampa River, many great restaurants, as well as crazy local flavor events like sheepdog herding contests & hot air balloon festival (early July).

If you can't make it to Steamboat, Estes Park is a nice place to go too. And if you are into resorts, The Broadmoor in Colorado Springs is gorgeous & overpriced, but you get what you pay for with amazing food, hiking, spa, and golf.

Happy travelling!