Booking your excursions in advance is good if you have a group you are planning with and have a particular service you want to use. Other than that I would wait until you get to the resort to book your trips off resort. You may find a group rate cheaper but on a small party the rates would be about the same.
I didn't think this question was crazy at all. I have been wondering the same thing myself. As my husband & I have a natural tan (lol), we r not planning on spending days on the beach. A couple of hours per day at best but, I am a pool person. We do plan on going on quite a few excursions.