Mersing Town Forest Park is a small green hill close to the centre, useful when you want shade and a quick break from the roads around the jetty. Trails climb through secondary forest, and the higher ground gives partial views back toward town and the coast.
It is not a wilderness hike, but it is a good short walk if you have time before a ferry. Go early in the morning if you can; the heat builds quickly and the forest is more pleasant before midday.