In Denver, I like to walk to the downtown breweries. I like to start at Great Divide Brewery for $1 for 1/3 pint samplers. Then head towards the baseball stadium and stop in Breckenridge Brewery. They have an awesome Vanilla Porter. Then you can have a really good time at 1Up, an arcade/pub place. Awesome beer, awesome old arcade games (home of the Donkey Kong World Championships). Next door is the Falling Rock Taphouse which has one of the largest beer selections in Denver (though they don't brew their own). Lastly head over to Wynkoop Brewing Company for their Chili beer or my favorite, their hefe. Wynkoop has a really nice dinner menu too and a pool hall upstairs.
I've never done all of those stops in one day (it would be a very long and very drunk day) but you at least know a lot of the options now.
Denver also has a really nice park by the REI headquarters called Confluence Park. Stop by REI to check it out (its cool to see even if you don't spend money there) then stroll along the river and if you're really adventurous (and don't mind a little urban runoff) bring a swimsuit and float down the whitewater play park.
At sometime while you're here, go for a hike (maybe on your way to or from Breck). Hiking is free, you get some awesome scenery and usually some cool history. If you're looking for something without too much elevation change (5,280+ feet is pretty rough for some people), I recommend hiking around the Red Rocks Ampitheater or doing Mount Falcon to see the "Summer Home of the Presidents." Both of those are nearby but really fun places to see.
I'd also recommend Bo Jeau's Pizza (there is one in Ft Collins) while you're here. It is a chain but they do a good job of showing off CO history (and they make really good Pizza!)
