Denver actually has a nice aquarium. There's also the botanical gardens, zoo, art museum, natural history museum, bums, etc...
How much time will you have?
Of course, there's the mountains. A 40 minute-or-so drive will get you to the top of Loveland Pass (provided it's open) where you can hike around and enjoy the wonderful view at 13,000 feet. I was just up there yesterday with a couple buddies. Wear snowpants/jacket. One of my buddies actually got a backcountry snowboard run in, and the snowdrifts have gotten somewhat deep (mid-thigh) in some spots.