Rent a car and travel the island yourself. We did this a couple of years ago. I think that we rented from Budget, a jeep for $59. Their office was in short walking distance of the dock. The island is only 12 miles long. It was great driving around on the left side of the road in a vehicle that was made in the US. We visited all of the sites and many areas that we would have never seen if we were on an formal excursion.
Try it - it was great. We are going back in July and I expect that we will do the same thing.