If you're going to be around CO Springs, which you will because that is where Pikes Peak is, you should see the Air Force Academy Chapel, the Professional Rodeo Hall of Fame and the Garden of the Gods.

If you get down to the south western part of the state, the Royal Gorge is cool and you should see Mesa Verde.

If you're around Aspen and Glenwood Springs you should see Hanging Lake.

If you end up out on the plains go see Beecher Island Battleground.

The area west of FT Collins up over Cameron Pass is pretty nice.

Don't forget to park the car and do some hiking. That is how I found most of my favorite spots.