Eagle/Vail (EGE) is about an hour from Keystone and Denver (DIA) is an hour and half. Those airports are the closest and I let airline price decide. Keep in mind that Denver can get busy so leave enough time to get through security when going back home. Are you planning to rent a car? We often use Colorado Mountain Express (CME) from DIA but have never used a van from Eagle. Might try these guys http://www.vailcoach.com/ Have fun!

One more thing. Summit County had a convenient *free* bus system that links all the towns and resorts. This comes in very handy if you don't have a car or enjoyed a little too much fun après-ski http://www.summitstage.com/

Vail/Eagle airport is the closest at about an hour and has limited flights.

Denver Airport is about 1 1/2 hours with numerous flights. Personally I would fly into Denver for the added flexibilty and airfare will usually be cheaper as well.

Keystone is a good location, personally I prefer Breckenridge but only because it has a better town and nightlife. As far as riding goes, opinions are split pretty 50/50 on which one has better terrain.

Either way you will have a great time