Another vote for lawn mowing. My house sits on an acre with few trees and the only outbuilding is a shed. I mow the lawn with an old-fashioned, no motor, Amish-style reel mower. It works my entire body. By the end of 30 minutes, my legs are screaming, my arms feel like jell-o, and I'm breathing HARD. But I'm already starting to notice a difference in my biceps. By the end of the summer, I'm going to be a gun show.