We rarely book an established cruiseline port excursion. Cruiseline offered port excursions tend to be quite expensive and structured. DW and I consider ourselves to be adventuresome and will usually just set out, on our own, for a day of exploring. Sometimes its planned, sometimes we just wing it. It all depends on the port. This message board and the cruise critic message boards have a wealth of information about what to do at different ports. I suggest you read up.
If you do decide to explore a port on your own, be sure you leave enough time to get back to the port before the ship sails. They wont wait for you, when you set out on your own. Also, make sure to arrange your return transportation ahead of time, be it a cab or something else. What could be worse then being on a remote beach at 6:15, finding out the last cabs leave at 6:00 and your ship sails at 8:00? This happened to us once. Thank goodness for cell phones, international cell service and American Express Global Assist