It depends completely on what you want to do, and where you want to do it.
Most water type activities, including
Castaway Cay, should be booked ahead of time. Even the glass bottom boat at CC was sold out when we got on the ship last Sept. I never realized it was such a hot thing.
Some places like St. Thomas, have "private" beaches all around the cruise docks. So if you want to "beach" it, you may want to go on a beach excursion where the vendors will leave you alone. Just soaking up the local air sounds great to me, and Key West has a tram that takes you around and you can get off and on as you please.
Grand Cayman and Cozumel have strips of touristy shops close to the pier, but the "real" stuff, you'd need a cab to get to. A lot of these tourist areas have hawkers that get paid to get you into a shop, and they use different degrees of "herding" to get you there. (Downtown St. Thomas and St. Maarten are THE WORST!!)
I wouldn't recommend renting a car at Grand Cayman, its THE most confusing place I've ever seen!! They drive on the left, and they import cars for both left-seated AND right-seated drivers. You get to an intersection and you don't know who's driving!!! Its REALLY weird!!
I wouldn't necessarily book something at every port of call, but if I saw something that looks like I'd want to do it, I'd book at cose to 60 days out as possible. Kelli