If you head South, you can see some of the things they've all mentioned. You could also stop in at Manitou and check out the sites. Garden of the Gods is nice to see and free if I recall correctly. Cave of the winds, Seven Falls, and a bit further South you can check out the Royal Gorge. Of course depending on the time of year you'll be visiting, those will be pretty warm. Rocky Mountain National Park is awesome and they've got that hotel where the Shining was made. Wife won't let me and the boys go inside. Staying near Denver you'll find plenty of really great food destinations and things like the Denver Museum of Nature and Science, the zoo and the botanical gardens are also cool.
If it were me, I'd do the loop sort of like timberman mentioned, but I'd try to maybe finish it off by swinging down South and hitting some of the places I mentioned before flying back.
If you get in a jam while you are out here, I'll PM you my cell. Just call and I'll do my best to help you out.
