Tree-planting in B.C. at any rate. We have lots of mountains. All day you're effectively climbing uphill with saplings and digging into the ground. At tree-planting camps (where you bunk) they feed you an enormous breakfast of flapjacks, bacon and eggs and you still lose weight and build up lots of muscle.

Never did it myself, but I knew someone who did. She had a fitness model's body by the time she was finished one summer.