Go check out Red Rocks Amphitheatre.
Drive up Mount Evans (or Pikes Peak, if you can take the time to get down to Colorado Springs).
Hike in Estes Park/RMNP



I haven't hung around Denver for awhile, but my understanding is that the area close to Coors Field that was previously a great place for restaurants and shopping (16th Street Mall) has now pretty much been taken over by homeless camps. Shops and restaurants closing down. Denver's kind of taken a bit of a dive, so...sorry not great recommendations for what to do downtown, but potentially look at...

Touring the U.S. Mint
Denver Botanic Gardens
Museum of Nature and Science


I have more recommendations for Colorado Springs if you want to venture down there, about an hour drive.