This was not just funny, but clever. https://www.youtube.com/watch?v=q3Fu2gTDMug is a video about building a tree house in Hawaii. This is a nice project, and if I wanted to spend my time traveling back and forth to Hawaii, it might be worth copying. The building is absolutely for tropical or at least warm weather only.
The film-maker had one great idea that I thought was just priceless. She rerouted her toilet tank supply hose to a sink, installed above the toilet tank. Every time you flushed the toilet, the supply hose squirted water into the sink, where you could wash your hands, and the waste water drained into the toilet tank, as it was originally meant to do, so you could perform both functions with one water supply.
I think that I would want to install a second sink, so I would not have to flush to use it, and I would perhaps put a professionally cut piece of glass on top of the tank to reduce evaporation and entry of bugs and debris. Just cut two holes, one for the drain from the sink and one for the supply line to go up to the sink.
What a great idea.
The only real drawback I can see, is the need to remove soap buildup from the inside of the tank on occasion.