Yeah i've been to st john's island. You'll be able to swim with
long lizards, chase chickens around, go fishing etc.. And right at
the top of the hill is some sort of accomodation area for large
groups which can be booked. It's very secure, surrounded by tall
fences. 
It was
orginally built to house the potential influx of illegal immigrants
from indonesia during the financial crisis but that didn't
materialise. Or you can book those small chalets near the jetty.
And yea it's run by sentosa.
