Thanks everyone for suggestions idea and tips.

Looks like these are the likely candidates:
(1) North to Estes Park, pop into Rocky Mountain National Park if weather allows
(2) West to Beckenridge, one way via US 285 and the return trip via I70 (or maybe across Loveland Pass)
(3) South to Colorado Springs, drive up Pike's Peak, Garden of the Gods.

Are these all reasonable day trips (would prefer to be based on Denver all the way) or am I being too ambitious?