Rent a golf cart for a day or two, tour up and down the island, checking out spots as you go. For example, cross the bridge to the north (unless already staying 'north') and check out Palapa Bar. I know you said you were interested in things other than food and drink, but Palapa Bar is a must see for view, at a bare minimum. Check out local entertainment at various places on various evenings. Entertainment runs from punta music to rock and roll, even some country and western, depending on which night, which place and which band.

You said you were aware that snorkelling is "rampant", but that said, the best snorkelling is done from a boat. For free snorkelling, check out the end of the docks. Ramon's dock is an in town favorite.

A fish and a bird can fall in love, but where will they build their nest?