Some people have more outfits than they can wear in a year; others don't have a clean change of clothes for tomorrow. You can help those in need by organizing a clothing drive with others also looking to pare down. Contact nearby agencies like the local branch of the National Coalition for the Homeless, your preferred faith-based group or a Salvation Army or Goodwill to ask about specific needs. Then invite friends and coworkers to donate their clean, gently used clothes, and to help you organize, pack and deliver them to the agency (or ask if they'll pick them up).
Calories burned: 251 for an hour of packing boxes, folding and stacking clothes.
Muscles worked: Legs (you'll be standing all day); arms, shoulders and back (hefting trash bags of cast-offs).
Karmic bonus: Your and your friends' stuff gets a second life.