Santorini - Affordable Caldera tour

I can't speak to the booking in advance part, but as far as the affordable part, there seemed to be a ubiquitously-offered one on the traditional wooden boats (advertised multiple places around town) that was pretty reasonable...I want to say 30 euro? Wouldn't require much advance research, I think, you'll see signs for it all around. We didn't have any difficulty booking it the day before, though our trip was in late August. Here are the tips I had previously written up about the tour:

Consists of a boat ride out to the new volcanic island being formed in the middle of the caldera, then a swim in the hot springs (warm sea water at the base of the volcanic island), and then if you take the longer tour there is a stop at a village called Thirassia. We chose the standard cheap tour on the “traditional wooden boat” that seems to be offered ubiquitously around the island. The boat was completely full, but we didn’t find it to be uncomfortable to the point of wishing we’d taken a more expensive tour. We did bring luggage locks for peace of mind, as we had to stash wallet/camera in our backpack during the hot springs swim (which some people elected to stay on the boat for). I’d strongly recommend taking the shorter (3h), cheapest tour that goes only to the volcano and hot springs rather than paying extra for the 5h tour with Thirassia. Much as I hate to be disparaging about any place, Thirassia isn’t worth your precious vacation hours IMHO. It is essentially a strip of somewhat run down restaurants. There is a town on the hill above, but they tell you that the hike up is effortful and that there won’t be much to do when you get there. They say swimming is an option, but nobody partook in this because the water is super cold and hard to get to due to big slippery boulders lining the waterfront. In the end, we decided that we wanted to save our appetite for a better restaurant in Oia, so we just grabbed a gyro and twiddled our thumbs for the two hours we were there. Bummer to waste two hours in the prime of the day when our time in Santorini was so limited. The complicating factor is that the shorter tour is only available out of Fira, meaning that if you’re staying in Oia you’d have to make the extra effort to take a bus to Fira and then make the long trek down the donkey path to the old port. The longer tour will pick you up directly from your hotel in Oia, which is nice, but then you just stand at Ammoudi (for us, it was an hour with nowhere to sit and nothing to do) while the shuttle goes back and forth picking everyone else up, so it’s still probably worth figuring a way to do shorter cheaper tour from Fira. If you do depart from Ammoudi, best photo ops are on left hand side at the start, right hand side at the end. Be aware that you’ll need to bring a few euros extra cash with you to get into the volcano area (they don’t tell you that when you pay for the tour!). Sturdy shoes are best, as you’ll be hiking 40 minutes or so up loose sharp volcanic gravel. Many (including me) did it in flip flops and I don’t think anyone fell, but lots of stumbling about. Expect to see a little puff of steam coming out of the crater (nothing dramatic) and a nice view from the top. No shade. For the hot springs, you’ll be turned loose to swim a pretty decent distance without any flotation devices, which was borderline hairy. I’m not a super strong swimmer. Our boat parked close enough, and there were just enough large underwater boulders to perch on that it somehow worked out, but if it been just a bit further or harder I probably would’ve struggled. The hot springs themselves consist of opaque rust-colored water that will stain light-colored bathing suits. We had lots of stubbed toes in the hot springs area, as there are underwater rocks and boulders everywhere but the water is opaque. Overall thought the tour was a good addition to our trip because it was something unique and different that you can’t do most other places.

All this being said, we are semi-budget travelers in our early 30s (meaning we don't stay in hostels, but we do try to keep expenses pretty modest on our trips), so if you're looking for something more luxurious, there are several other boat tours available on the island :)