So what is there in Phi Phi Don, than you can only do if you stay overnight?
The answer is the night plankton trip.
It's an afternoon, evening boat trip, similar to other trips, but at the end it is dark, and you go see the plankton light up. For this I went with a group, and we did things I'd done before, more snorkling, more visits to Maya bay.
For some extra excitement they jumped off a cliff into the water.
A little extra thrill to make their holiday more memorable, but for me the high point was the plankton. You are taken to a dark cove on Phi Phi don, after sunset. You snorkel in the darkness, wave your arms around and the plankton show as little bright spots of light.
Like my many firefly trips, this is insanely difficult to film! The underwater camera insisted on turning on the light on underwater and I simply couldn't film them. The camera couldn't catch the tiny flashes of light.
You should go see them yourself, and for that you'll need an overnight stay at Phi Phi Don!