[Reinhard] has a great collection of DIY SCUBA projects. One interesting hack he put together is a soft start circuit to improve the life of the bulb in a flashlight. Despite LEDs getting cheaper, MR16 halogens are dirt cheap and put out some serious lumens.
Most people fear refilling their own ink cartridges. [Skipbreather] made his own CO2 scrubber refill for his rebreather. (Modern rebreathers are damn impressive.) The build is fairly straight forward, but requires some machining. I had to laugh when I found out that part of his build involved toilet seat hold down bolts.