Welcome to Pulau Kakaban & Pulau Sangalaki
Pulau Kakaban is famous for its inland lake (admission by donation) where bizarre stingless jellyfish have evolved. It's a 10-minute walk through the jungle along a boardwalk to reach the lake jetty, from where you can snorkel (don't use fins) in shallow waters amid an ethereal swarm of jellyfish, some as tiny as your fingertip. If tides permit, snorkel through Kakaban's tidal-cave tunnel to a hidden outcrop of protected pristine coral.
Pulau Sangalaki is a real castaway island with decent diving and a small resort, but is known primarily for its consistent manta-ray spotting. Trips to Manta Corner will usually result in numerous sightings.