I grew up just north of Boulder, in Longmont. Haven't been back for quite a few years, but here are some ideas:
Estes Park is very fun, but very busy in summer. Lots of cute shops.
Rocky Mountain National Park and Trail Ridge road up to the great divide. (Prepare yourself for a lack of oxygen!)
Celestial Seasonings tea factory tour in Boulder/Gunbarrel.
Red Rocks amphitheater
Flagstaff mountain in Boulder- scary drive, but spectacular views at the top!
Casa Bonita in Denver is a cheesy but fun restaurant with cliff diving inside.
Denver zoo is good.
Down in Colorado Springs there is the Air Force academy and Focus on the Family headquarters. FOF has a great tour and you can visit Whit's End! Also the Garden of the Gods is down there, but I've never been.
Pearl Street mall in Boulder is a very interesting cultural experience. Probably don't want to do this with young children.
Have fun!!