Sometimes the simplest designs are also the most brilliant. This portable hot tub couldn't have a more simple design -- the water is warmed through the spiral coil with a wood fire and then sent back into the tub. It's called the Dutchtub and was designed by Floris Schoonderbeek as part of his thesis in the product design department during his time at art school in the Netherlands.
The designer wanted to create links between indoor and outdoor living, the city and the country and help people to gain independence from their bathrooms to discover the natural world around them. There's a whole website dedicated to people's pictures of boundless bathing. The site has everything from pictures of the best roof racks, most scenic, most complicated deliveries, coolest kids, hottest hunks and tubettes, most urban and most outdoor pictures.
It can take anywhere from 2-5 hours to heat up the water in the tub, depending on the temperature of the water, fire and air around it. But with a little planning, I think it would definitely be worth the wait. There is no electricity required and no chemicals needed either. There is a plug in the bottom to drain it and this water can be used for gardening or recycled into a grey water system. You can even contact the company for project details on grey water systems. The tub can be cleaned with a wet towel and green cleaners and the spiral itself can just be rinsed out.
It's tough enough to withstand winter weather outdoors although you do need to make sure the water is drained from the tub and the coil or you'll end up with a very expensive ice cube. It runs around €4950 which is approximately $6500 USD. This is pretty comparable to the price of a more traditional hot tub but you don't need to pay electric bills or buy chemicals either which will save you in the long run.