I would recommend booking at least some excursions as early as possible. Certain excursions like parasailing on CC almost always fill immediately. Other excursions, like beach days, have tons of open slots and do not fill as fast.
My recommendation is that if there is something you really, really want to do (like parasailing, dolphin encounter), I would book as soon as possible.
Can't remember the cancellation policy but you might want to check that out.