Keystone's summer identity is less known than its ski reputation, but the resort makes a genuine effort to stay busy from June through September. The gondola and some chairlifts run throughout the summer, serving a mountain bike park and the hiking trail network above treeline.
Mountain biking is the dominant summer activity at the resort. The Keystone Bike Park runs lift-served downhill trails from late June through early October, with terrain ranging from wide-open flowy runs to technical rocky lines. The A-Line trail is the signature run: consistently bermed, fast, and approachable for intermediate riders. Rentals and full-face helmets are available at the base. Trail access without a lift ticket is also possible, with cross-country routes connecting into the surrounding White River National Forest.
Hiking at Keystone reaches into genuinely alpine terrain. The summit of Dercum Mountain sits above treeline and offers 360-degree views of the Tenmile Range, the Gore Range, and the Continental Divide. The Peru Creek Trail is a popular route into a broad alpine valley with old mine ruins and good wildflower displays in July and August. Difficulty ranges from paved lakeside walks to exposed ridgeline hikes.
Lake Dillon is a short drive away and anchors the water sports scene — kayaking, paddleboarding, and sailing from the Dillon Marina. The circumference trail around the reservoir makes a long day hike or a solid bike ride.
Golf at Keystone Golf is a nine-hole course that operates in summer at around 9,300 feet. The altitude means the ball flies noticeably further than at sea level, which players either find amusing or disconcerting.