D.D.
Denver is really a great city. I live in a north Denver suburb. In downtown Denver there is the capitol building, which my boy enjoyed as early as 13 months.... it's big! The Denver Library has a huge children's room. There is the Denver Art Museum, the Denver Botanic Gardens, Children's Museum, a walk along the river is alway nice. My boy has enjoyed days in downtown with M. since he was pretty little, and really started to enjoy it when he could walk (he started late, at about 16 months).
Boulder is beautiful and in the foothills. Expensive! There is Chataqua park with wonderful, easy hiking trails. And El Dorado Canyon State Park also is gorgeous and has easy walking/ADA accessible trails. You can always see climbers at El Dorado also.
Go into the mountains on I-70 to Georgetown and take the train up to Silverplume. Great little train ride and Georgetown is a fun walk down an old mining town main street. Cool specialty shops and a couple of great ice cream parlors.
Estes Park is the gateway city for Rocky Mountain National Park and it is a really neat little town. We like to stop in Estes Park for a walk on our way into the national park. Of course hiking/walking opportunities at RMNP are wonderful. It is about an hour drive from the Denver/Boulder area.
Hope this helps just a bit.
Yes, TwinMama is right. You absolutely MUST drink LOTS of water when you are here. Altitude sickness will give you a head ache, make you dizzy, and because it it hot here, you'll overheat fast. Even in the mountains, the summer time is HOT. Yes if you are in a shaded area things are fine, but just don't neglect your water.