Ran the calculations some time ago, and donating a half liter of blood gets rid of about 500kcal, which I can easily manage to do in under half hour not counting the paperwork. I think the only more calorie-consuming activity I can think of is liposuction or mutilation of body parts. Not that I recommend either, just a random thought.
Routine blood donation / letting supposedly reduces concentrations of lead and certain “forever chemicals” too
That’s apparently disputed/not as much as proponents claim, but donating blood definitely works on people with too much iron in their bodies.