Been there twice. Once did the usual hotspots by private car tour, the second time most of the same but by catamaran and open-air van, including snorkeling and homemade lunch at a private home. Stops included the volcanoe, waterfall, mudbaths. If you check out port of calls at CC, the operators show up. These are all day tours; don't know if you will have young ones with you. We were 2 adults. After a lot of cruises, I DO feel comfortable using non-ship excursions to save money and not worry so much about missing the boat. If you are too, a google search could find you what you want to do by a third party as well.
Ironically going on the S Car cruise in Jan, returning to St Lucia. This time we're trying fishing!!
Good luck and have fun!