You can also plan your own excursions, and put them together for less than if you booked it through the ship. Especially with the beach excursions, you can almost always do it yourself for less. When you do this, you also get to go at your own pace and not have to keep up (or wait for) the rest of the group.
One resource I've found helpful for this is Frommer's Caribbean Cruises and Ports of Call. This book is updated every year, and has a chapter for all of the most popular Caribbean ports, with contact information for tour vendors, as well as shopping, dining, and transportation information.
Also, you can book your own excursions through a third-party vendor, such as Shoretrips.com. I've never personally used this site, but they do list many of the same excursions as are available from the ship, but for a bit less. I've often heard people report that when they booked through Shoretrips, they end up on the same excursion with people from the ship, but paid less to do so.