If you're here for a weekday, then give skiing or snowboarding a try! Buy discount lift tickets at area grocery stores - there's a King Soopers right near Regis. Loveland is one of the closest resorts, but it's also steep and windy there. If you're a beginner, I'd suggest Keystone or Copper.
Bicyclist? Rent a bike at REI Flagship store and enjoy one of the largest bike path networks in the west - I suggest the bike path along Cherry Creek - you could bike from downtown out to Cherry Creek Reservoir and back.
Bookworm? The Main Branch of the Denver Public Library is one of the best public collections in the state
Art Lover? Try the newly-expanded Denver Art Museum, or the Museum of Contemporary Art.
Free Art Lover? Denver's chock full of public installations.
Beer Lover? We're the microbrewery capital of the west. Send yourself on a mini brewery crawl...Wynkoop Brewing is one of the first microbreweries in the state and used to be owned by our Mayor.
Shopper? Hit the tawny Cherry Creek Shopping District.
Music Lover? Zoom out 6th Ave Fwy to C-470 South and Exit at CO-8 Morrison and head up to Red Rocks Amphitheatre. The stairs from the bottom of the venue to the top are a workout!!
let me know if you need more suggestions - and enjoy your time in the Mile High City!