There's nothing new about hot tub rental. It's been done to death. Most burn propane. We're going to burn wood in a snorkel type stove. There are many old fiberglass tubs available for free. Start with those. Most have dead heaters and pumps. The simplest thing to do is fiberglass over all openings, so that it's just a giant soaking tub that is dead simple to clean.
This sort of tub might have to rent out for half or a third as much as fancy jet tubs. That's fine. Your purchase, fuel, cleaning, chemical and repair costs will be minimal. Put it on a nice trailer with a good sound system and you're in business. Make customers watch a DVD on how to run it.
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--- DJ Tubbs ---
If I were doing this, I would make it part of a party package where I would be the tub operator, MC and DJ. This would protect the tub from improper firing and I'd get to meet lots of drunks.

If you are a permie by day and an aspiring musician or DJ by night, you'll starve without a hot tub. Around here the fancy tubs go for $350 for a weekend and $250 for a one nighter. Charge $200 a night for a good old tub fired with wood and stick around for the party. You need to get there a couple hours early, to set up and fire the thing. You might get home by 4 am.
Given a choice between a DJ that they know who shows up in a battered van and someone they've never heard of who shows up with a hot tub in tow, I guarantee that most rednecks will choose the tub guy.
You're welcome --- Don't turn into a drunk.