Hi there,
There are a number of dive centres on Phangan, we recommend Sail Rock Divers to all of our customers who head over there. Phangan's dive centres are scattered fairly evenly around the island, so where you stay really depends on which centre you choose to dive with (Sail Rock Divers are based in Chaloklum). Your dive centre will also be able to recommend suitable accommodation options for you in the area.
Dive site wise, Sail Rock is pretty much the only option, unless you want to pay more for speedboat transfers to Koh Tao dive sites.
In terms of other things to do, some of the waterfall hikes are very cool, and we highly recommend the Just For Fun Canopy Adventure too.
Happy holiday planning