The only thing that comes to my mind is drowning. Depending on where you do it, there's even a chance they won't find your body. Alternatively, you can go to the mountains during a big and frosty winter and let the temperature do its thing. But it's a bit risky and you need to minimize the chance that someone will find you. Maybe when they find the body, they'll think you're lost or something. Of course, I don't encourage anything.