If you're not setting something up yourself with a local person/company, then just hang tight.
The excursions that Jam Cruise offers are typically snorkeling and sightseeing kind of things cruise lines always offer, but some of ours include guest artists and stuff. You don't sign up for those excursions until you get on board.
Then there are the Positive Legacy excursions, that generally involve some type of service aspect (cleaning up beaches, planting trees, etc) with lunch and entertainment by locals and artists we bring on shore. These are usually AWESOME (IMHO/YMMV) and I would highly recommend trying one out. They will be announced pre-cruise and you will be able to sign up/pay for those excursions pre-cruise as well.
Or you can set something up yourself with a local person/company, as I mentioned... or you can just disembark and wander, see what peaks your interest
lots of options!