Payoff Everything is concentrated when you make a one-day double crossing of the Grand Canyon, like I did in May. Your fuel: GU and 5-Hour Energy offset the 20,000 calories you’ll burn—and keep your pack light. Your attention: Tiny scorpions come out at night, blunting the fatigue that could send you stumbling off a cliff. And most of all, your adventure: The physical challenge is highly satisfying, and so is the scale, but the real reward is covering so much ground that you sear every ecosystem and geological wonder onto your mental hard drive. —Jonathan Dorn
Do it The vaunted rim-to-rim-to-rim, or r2r2r, is a burly 44 miles and 22,000 feet round-trip via the South and North Kaibab Trails. Alternatively, return via the Bright Angel Trail—it adds three miles, but cuts 500 feet of climbing and offers water and shade options (the South Kaibab doesn’t). Don’t attempt the r2r2r in hot weather. Go in April (more water in side creeks) or October (heat fades).