I book an excursion about 90% of the time but don't feel obligated to book now. Read up on what's available online and think about what you like. You can book when on the ship although you risk having something already full and who wants to stress planning when on board. Cut loose and forget about it then
Things with small groups are what tend to sell out like swimming with the dolphins or scuba diving. Beach excursions are easy to get but you can always choose to get off the sihp and just tell the taxi driver to take you to a nice beach. Make sure you negotiate the fare beforehand though.