The main attraction is the lagoon, which is difficult to access unless the weather is very dry.
Sempu is famous for its lagoon, which is sometimes called the Indonesian version of "The Beach" (the Leonardo DiCaprio movie).
The water has a slight fragrant smell, perhaps from the mangroves. The water is clear on the mainland beach (Sendang Biru) but more muddy around the mangroves on the island.
Flora and fauna
Sempu is surrounded by mangroves.
About 3 hours drive from Malang.
Large groups (e.g. 50 people) will need to arrange permission - this can be arranged on the day, but seek information before arriving.
Another option to discover Sempu Island is by boat and also there are many tour & travel agents out there.
Walk to the lagoon. This can be 1 to 1.5 hrs each way in dry weather, or a gruelling 3 hours or more when muddy.
Warning: The track tends to stay muddy for a couple of days after rain, and you can expect to either go barefoot, or struggle not to lose your sandals in the mud. Also expect to come back with scratched or cut feet. To avoid this, the ideal time is well into the dry season, e.g. August.
Large fresh fish from the fishing boats.
The warung at the entrance to the park, at Pantai (beach) Sendang Biru, makes fresh food daily, at regular Indonesian prices (you can eat well for under a dollar). If you want something cooked fresh, try walking or driving along the track away from the entrance - one of the warungs may be able to help you.
If you want ikan bakar (grilled fish) you must order in advance (at least a couple of hours) at one of the warungs.