
Team up with Grand Canyon Trust Volunteers on a free six-day backpacking trip to remove invasive trees from the banks of the Paria River. You'll work your entire body and burn 500 calories per hour. Go to gcvolunteers.org for information.
Take to the woods with the American Hiking Society on a weeklong vacation in a state or national park ($245, including lodging and meals; americanhiking.org). Between hiking and clearing paths, shoveling and repairing walls, you'll work off 480 calories an hour.
Give the seafloor a sweep during the Ocean Conservancy International Coastal Cleanup. On September 20, volunteers will scuba dive for debris—torching 240 calories per half hour in the process. For information, visit oceanconservancy.org.
Gather your pals for a day of canoeing for the American Rivers National River Cleanup. Log on to americanrivers.org and you'll get free bags to collect trash that ends up in our waterways. Bonus: Paddling ticks away more than 200 calories per hour.