I live about two hours from Denver. Ditto to the comments on the zoo and the museum - both are really nice and you can easily spend an entire day at each.
Tattered Cover is a really cool bookstore (two locations, both are fun) if you like that sort of thing.
The Cherry Creek Mall area and the 16th Street mall are fun if you like that sort of thing. (but what good mustachian likes fancy, overpriced malls?)
The Ogden Theater is fun, but I don't know what is booked while you are in town.
Budweiser has a big plant in Fort Collins that has free tours (and free samples!). Sometimes the horses are there, too.