We rented a car for the day. The only problem is that the car rental closes a 5pm so you have to take a taxi back to the port after dropping off the car. This was a bit unnerving because you had to worry about calling a taxi in the dark but there were luckily two taxis just sitting at the car rental lot waiting for cruisers to arrive. It is an extra expense that you didn't count on.
I booked this company on my own for 3 of us and we were joined by 4 from another ship that was in port. After a wild and crazy van drive to the other side of the island, we did an off road Segway ride through the forest and to a beach. After I got the hang of it, it was a lot of fun. It was cheaper and longer than the trip offered by the cruiseship.