My family has been vacationing in Colorado for over three decades and one of our favorite places is Estes Park. It is about 1 1/2 hours from Denver and adjacent to Rocky Mountain National Park. There are a lot of cabins/houses that you can rent but our favorite place to stay is YMCA of the Rockies. There is something for everyone! I like to describe it as a cruise on land....lots of activities for all ages and all abilities that are included when you stay there like mini-golf, hiking outings, concerts, family sing-a-longs and other programs, crafts, and activities for kids like orienteering, water discovery, beaver walks, nature walks, etc. For a fee you can also do fly fishing, horseback riding, river rafting excursions, etc. Basically, there is enough for everyone to choose to do things together, do their own thing and a combo of both. We love it and my kids have been going there since they were born!
Here is the website in case you want to check it out:
http://www.ymcarockies.org/
We stay at the Estes Park center in the summertime. They also have Snow Mountain Ranch, which is in Grand Lake (other side of the mountain) and while they have activities, they are much more low-key and don't offer as much in the summer (they are great in winter though and close to skiing!)
Main thing to know is they book early so have some dates in mind when they open up summer registration so you can hopefully get your first choice. You can stay in lodges (like hotels) or reserve cabins where you can cook your own meals, etc. (or you can still go out or take advantage of the cafeteria they have there). Good luck!