Our favorite place to stay, by far, has been the YMCA of the Rockies. It is BEAUTIFUL, and it has just about every outdoor activity you can think of included in the price for camping. They even have an indoor swimming pool and a roller-skating rink, and my son learned rock-wall climbing!
I think the only things we had to pay extra for was mountain bike rentals, the rock-wall climbing, and horseback riding.
Camp Chief Ouray is the only part of the YMCA that offers camping, and it is on the "other" side of RMNP, near Granby, CO, which is a few miles from Winter Park. There are plenty of fun things to do in Winter Park, also, including alpine slides and great mountain biking trails. Estes Park isn't a SHORT drive from there, but in the summer, you can drive the GORGEOUS road through the park, which winds its way up to the Alpine Visitors Center (a great place to get out and look around in the alpine tundra). It took about an hour, hour and a half to get from our campground to Estes Park. There are plenty of things to do in Granby, Winter Park, and Grand Lake, too, though, so you don't HAVE to go to Estes Park.
Have a great time - that area is one of our favorite vacation destinations.
P.S. The rates are REALLY reasonable there, and if you're a member of the YMCA at home, I THINK you can get a discount!