The silliest thing I built was a hot-tub water heater system for one of those inflatable Intex hot tubs. Running the heat off 1500W was stupidly inefficient, so I went out and acquired one of those tankless propane-powered hot water heaters and rigged it to a commercial RV water pump and a 20# propane tank to circulate water into the hot tub that way. It heated the tub up from ~50* faucet to 104* hot in about 3 hours, instead of closer to 30 on 120V power.
The other thing I built is more of a lifehack living with sleep apnea. My CPAP's humidifier failed, and instead of replacing it with another proprietary humidifier, I bought an external CPAP humidifier[1] and then realized I could, with some clever plumbing, rig up a water pump from an external gallon jug of water to automatically re-fill my humidifier in the morning at the push (and hold) of a button. I took an oxygen-insertion tee, 12v pump, and some tubing and a check valve and attached the pump to the oxygen line on one of the inlets of the humidifier chamber. When you push the button, water flows through, through the check valve, and immediately down into the chamber.
It's been tweaked a little bit since then but it's my FAVORITE thing on my nightstand now. (And yes - that is a recalled DreamStation 1 - I personally opened it and repaired the defect foam myself. I don't trust the replacement device they sent me any farther than harvesting the motor from it when my old machine fails.)
"Redneck hottubs" are a relatively common thing and you can get some amazing looking ones, from a fire under a cast iron bathtub to a complex system involving miles of copper tubing and burn barrels.
The other thing I built is more of a lifehack living with sleep apnea. My CPAP's humidifier failed, and instead of replacing it with another proprietary humidifier, I bought an external CPAP humidifier[1] and then realized I could, with some clever plumbing, rig up a water pump from an external gallon jug of water to automatically re-fill my humidifier in the morning at the push (and hold) of a button. I took an oxygen-insertion tee, 12v pump, and some tubing and a check valve and attached the pump to the oxygen line on one of the inlets of the humidifier chamber. When you push the button, water flows through, through the check valve, and immediately down into the chamber.
Here's a snapshot of an early test: https://i.imgur.com/vund22E.png
It's been tweaked a little bit since then but it's my FAVORITE thing on my nightstand now. (And yes - that is a recalled DreamStation 1 - I personally opened it and repaired the defect foam myself. I don't trust the replacement device they sent me any farther than harvesting the motor from it when my old machine fails.)
[1] https://www.fphcare.com/us/homecare/sleep-apnea/cpap-devices...