If you are feeling a bit claustrophobic staying in Boulder there are lots of things to do in Denver.  Going out in Denver will get you away from the Boulder college scene.  The beer may be more expensive, but there is a bigger variety of clubs (dancing, piano bars, martini bars, etc) and the crowds are completely different.

-Sporting Events.  Denver is home to a variety of professional sports teams:

    Colorado Avalanche - Hockey

    Colorado Rapids - Soccer

    Colorado Rockies - Baseball

    Denver Nuggets - Basketball

-Music Shows.  There should be something for all tastes.  Check the Westword for events.

-Theater. The performing arts center has a variety of musicals, plays, concerts.

-Amusement Parks.  Denver is home to Six Flags Elitch Gardens as well as WaterWorld.

-Shopping. Denver has an area similar (but larger) to Pearl Street right near the bus station, known as the 16th Street Mall.