It depends on how far you can get away from the city. The zoo is pretty wicked place if you want to stay in, but if you've seen one zoo the better chance you've seen pretty much every zoo. I believe you might be at the beginning time for wildflower season, rent a car and go up into the mountains if that is the case. Just before you do, Google wildflower blooming in Colorado because different areas have different times (some start in April, most in June/July, and even in August).
5 Colorado Wildflower Hikes | Colorado.com
A Guide to Colorado's Spectacular Wildflower Season | 5280
With how much snow we've gotten this year, I wouldn't expect wildflower season to start in early April. I'd anticipate a lot of hiking trails the next two months to be either a posthole nightmare (higher elevations) or a mudpit (lower elevations). You might be able to find some dry hikes in the flatlands near Boulder or south of Denver.
The Denver Museum of Nature and Science is a pretty cool spot, as is the Botanic Gardens. If you don't mind a bit of a drive, Nederland and Estes Park are fun towns to spend a day in, but like I said before, it's not the best time for hiking up there. Colorado Springs has Garden of the Gods and Cave of the Winds.
Skiing season is still in full swing, and the conditions have been great this season. Most of the nearest resorts will still be open (A Basin, Keystone, Breckenridge, Eldora, Winter Park). Skiing obviously isn't cheap though.
