There is something luxurious and decadent about the concept of bathing outdoors. Outdoor tubs and showers are fantastic ways to feel like you're at a resort and take you out of the indoors/computer/tv screen rut.
A beautiful nonstop flowering Hong Kong Orchid tree in my backyard had me recalling a daydream I once had of a luxury backyard with a bathtub.
The more I thought about this, the more I realized that it could be a mutually agreeable arrangement.
The tree needed carbon dioxide from me, I needed oxygen from it. Its fragrance and beauty gave me pleasure and me draining a tub full of water and plain epsom salts (helps blooms form), would give us a symbiotic zen.
A private nook in the yard where you can fill a tub and empty it, watering a tree or a garden (don't use any soaps that can harm the plant life), is a most decadent luxury.
Here's some ideas that I think are awesome for privacy, skywatching, soaking, a side table for a tray with wine and accessories, lights, folded fluffy towels....