KEYSTONE,CO. Keystone offers the largest amount of terrain in Summit County, CO. It is also home to the longest ski day in Colorado. Top ranked terrain park in the state and one of the best in the country. First Time Guests At Keystone, you'll find everything that makes a perfect vacation all in one place. Offering unique activities like night skiing, KAT snowcat skiing and riding the bowls, tubing and dinner sleigh rides it's all here at Keystone. Brokered And Advertised By: Listed On Inc. Listing Agent: John Fothergill