Want the luxury of a hot tub, but don’t want to burn a lot of natural gas, or coal to heat up that water. Check this out.

Step by step instructions on how to build your own here. Now it might not be the cleanest thing ever (burning wood makes particulate pollution) but it will not add new co2 to the air (the co2 from the wood, is the same that the tree sucked up) and if you bury the charcoal left over you might be able to enhance the soil and actually store carbon, making this a carbon negative fuel. Either way it looks like a fun project (and it uses recycled materials).