Check out the Colorado Mountain Express (CME) if you don't want to rent a car. If you go that route, I'd recommend Breckenridge or Vail as they have more stuff in their villages that is walkable or easy to get to. If you do Keystone, you'll probably want a car to venture to silverthorne or Dillon. As of now, Vail has the most open terrain but everywhere is supposed to get snow in the coming days which will open more of the mountains up. I was at Keystone yesterday. It was fun, but they were pretty limited on runs that were open. Three days in a row of skiing is pretty ambitious- you might want to plan something else for the middle day unless you're in really good shape and used to the altitude. Drink lots of water- before I moved out here I would come and visit every year and think it was a good idea to only drink adult beverages and no water. Altitude sickness is no joke and will wipe you out for a few days, so stay hydrated. I'm not exactly sure what the price is for lift tickets with the airline discount, but I've heard that's the cheapest route unless you're going to get 8+ days on the mountain this year.