We wrote on this topic years ago and it's something we do every summer but this year I'd like to take it to the next level and actually set up a special area on our back deck for outdoor bathing. First up is to purchase a large galvanized tub. Another nice touch would be to put a teak bathmat next to the tub. Drill in a few hooks for fluffy towels and you're all set.
If you have the room, you could go even bigger with this idea and buy a galvanized stock tank to use as a small swimming pool. See how Kathy (aka Waldorf Mom) did this a few years ago.