I've built a couple neat things. A solar tracker that uses physics (heat and gravity) to track the sun instead of electronic sensors and motors, and a small evaporative cooler for my nephew's skoolie.
Hmm, I should probably get around to finishing that page one of these days....
You can see a store-bought window mounted swamp cooler just above the block wall, the one I built is the white box on top of the bus. The idea was to build one with higher effectiveness that didn't have to be removed when they are traveling.
Page with build details
If we had built it as designed, it would have worked even better. Moral of the story, don't mention prices to your wife when building cool stuff. (pun intended)
And just for fun, a top bar bee hive made from reclaimed lumber
My opinions are barely worth the paper they are written on here, but hopefully they can spark some new ideas, or at least a different train of thought