My wood fired hot tub! If I ever get it built, it's currently in the "Staring and planning" stage. A piece of metal for where it goes through the roof, probably weld a piece of heavier pipe to the top of the stove for the pipe to attach to, along with welding the lid on and filling the extra holes. The thin sheet metal will be a bitch to weld, but I pulled that off one time before.
Going to wrap a coil of copper pipe around the stove, then one end will go high, one end low, into the side of a water trough, like I use for a compost bubbler. Maybe some rock or brick around the whole stove and copper pipe, to hold in heat.
I figure if I can heat the water in the hot tub, it will radiate back at night, and I can finally quit burning propane. Scored some red wood and cedar boards to make a cover for it with, so I can keep humidity in check. The stove is an old smoker I picked up off the curb last summer, had a free sign on it. Mostly used stove pipe. It's class B gas vent, so we will see how it holds up. I've used stove pipe made of less, so am hopeful that I don't burn myself down!!