We spent a week in Breckenridge a couple summers ago and had a fantastic time!
We all loved the Breckenridge Fun Park. The roller coaster, luge, ropes course and other stuff was an expensive day but fun. We even went back another day and didn't buy a pass - just played in the snow up there. We went white-water rafting too, but it was August and the river was pretty calm. The town is adorable with great restaurants. One thing I wish we had done is rent bikes - either ride down from the peaks at the fun park (it looked like there were paths that would be fine for novices like us), or there were companies that would drive you out of town and you could bike downhill back into town. I think the kids would have loved it.
If you're driving in, you'll probably do better than we did with the altitude - we flew into Denver and drove right up to Breckenridge that day. We're from the Midwest and it was too much for us. We should have taken at least a day in Denver first.
-Christine
DD, 9/07
DS, 5/09
and a very tolerant pup