Used to live there! Lots of fun stuff to do in Denver:
16th Street Mall, Molly Brown House, Eat at the Brown Palace, take a trip to Golden to see the old mining town and Coors Brewery, Elitch Gardens is a fun amusement park to spend a day at, Denver has a zoo, museums (the natural museum is fantastic) and a botanic garden. If you love books and want to spend a good part of a day in, what I think is the BEST bookstore, check out the Tattered Cover. It is amazing!