We rented a car and had a great time touring the island. You should check your auto insurance - changes are you are not covered in a foreign county. We looked at all the rates on the net and then picked the best one of those rental car agents we knew.
We had heard that the rates were higher on the day of the trip, so we wanted to be covered. In actuality, we ended up with a better rate when we walked in, just the luck of the draw I guess....
Have fun....
Driving hints.....There are maybe 3 stoplights on the entire island. The port town is gridlocked most of the day so be patient. When driving out of town, there are no lights, 3 and 4 way stops. Most of the island roads are 2 lane and once you leave the traffic - no problem. Quite a nice drive. It is an island, so there is not much room left for signage...We use the 180 degree turn around......Just warn travelers ahead.
We like to explore non-touristy spots and this was perfect. However, you will not have time to "do everything" in one day....
Enjoy!