Diving, partly because of money, but I’m also not sure about the medical requirements.
Also flying, same reasons but money is a bigger factor there.
22 years ago I went to Madagascar as part of an organised group. We were scuba diving off the reefs in the south, cataloguing and counting fish and coral species.
I racked up over a 100 hours underwater in 2 months. I loved it. I was going to get my PADI training, then spend the rest of my live living in the tropics taking tourists our on boats.
I got back to Scotland and I haven’t even snorkelled since, let alone scuba’d.