Swimming with sea turtles (Maui, USA)
Right across the street from my one-week-every-other-year timeshare near Lahaina on the island of Maui, there is a small beach whose main attraction for me is that there are often sea turtles there and so you can snorkel with them.
Make sure to not harass, touch, or annoy them in any way as they are protected, but if you just float there or swim in their general area, they’ll just do their turtle thing happily with you watching them from a distance.