Yeehaw! A good 3000ft of downhill and 50 miles of mountain bike trails at Snowmass guarantee good times in the summer. Most trails are accessible from either the Elk Camp Gondola or Elk Camp Lift. Some of the classic cross-country trails, such as Government and Tom Blake, don't require a lift ticket.
Check out www.rfmba.org or ask around at rental shops for other good singletrack rides in and around the valley.