Keystone Resort enjoys a reputation for being a solid family mountain in winter, a laid-back place where many of Colorado's kids learn how to ski. But come mid-June, Keystone turns into a pretty hard-core mountain-biking mecca. With 55 trails comprising 100 miles' worth of lift-served and famously technical singletrack — complete with rock gardens, natural and man-made obstacles and plenty of places to get air — Keystone features some terrain that gives even the most gnarly mountain biker pause, all for $42 for a day pass (multi-day and season passes are available, too). The black and double-black runs offer serious speed, and the newer TNT section, an old logging road turned into a berms-and-jumps ride, has been revamped with sheer drop-offs in the smoother sections and "The Yacht," a wooden structure that gives you one last big move at the end. There's also a skills park that allows riders to practice on jump trails with rollers, logs and rocks at less-steep pitches than in the main park, as well as the Drop Zone, a series of ridgelines perfect for free falls from five to fourteen feet. There are a few beginner- and intermediate-level trails here, but for the most part, it's experts-only — and while you don't have to wear body armor, you'll probably want to.