Pick a sport–any sport–and enjoy this day of active rest.
Mountain biking An easy one-hour ride, especially if you spend time out of the saddle, engages your core and works your shoulders and chest, employing the same muscles you use when you climb with trekking poles.
Yoga Getting some mat time builds balance, core strength, and flexibility–and teaches you how to control your breathing, a skill that can make a long climb feel easier.
Running Go for a mellow jog on varied terrain. Aside from boosting cardio endurance, the uphills will work your calves and the downhills will strengthen your quads.
Spinning Hop on a spin bike and do intervals, alternating between high resistance (like climbing a steep trail) and low (easing up when the trail levels off).