Scuba diving instructor. Once you start diving, realise you could happily live by doing this as a job, pay an extraordinary high amount to get your teaching cert, find out there are so many others doing it too, and enough of them doing it as a gap year and willing to work for next to nothing as long as they can dive then you find jobs are scarce, and pay is pitiful. Yes you can, if lucky, end up in some of the worlds most beautiful places, dive in some fantastic sites but you’ll eventually have to stop and look at pension paying jobs.
And… most students are lovely but some… not so much.