Crsted Butte Colorado is amazing
http://www.visitcrestedbutte.com/
The skiing is probably the best I've ever had, I went the first week in April for about 4 years. Lift tickets were free back then, not sure if they are anymore since the X-games came through. I can't speak for montain biking because I've never been in summer.
Edit: didn't realize this had already been suggested. Should have read. Crested Butte is awesome!!!