Home theater room in the attic. A more do-able idea than those other fancy home theater rooms! Much cozier and warmer than the individual recliners in rows that you see in a lot of fancy home theater rooms
Outdoor shower/water day for kids.Could easily be made with hollow box uprights, run the water tube through one and the top hollow box with a PVC tube that you drill holes in for the water to run out. Faux paint it to look like copper, really pretty easy.
Reuse an old tree to make a log pathway in your garden. Good use of "yard" trees and other wood that can't be milled; works much better in climates without termites. Good drainage under the log rounds will make the path last longer.
Okay, I know this is a spa and not a house. But if there was a "waterfall" in my house, with a small pool to hang out in, in a room with tons of natural light, and all that good stuff.I would never leave my house.