We purchased the multi-day bus passes and had a great time exploring. In St. Georges, make sure to take a swim at Tobacco Bay -- the fish are amazing to swim with. People we were with brought rolls from the ship and held the bread above the water and the tropical fish jumped right out of the water to get it. It was awesome.
We signed up for helmet diving, but they canceled it due to turbulent seas. We went to the perfumery, the Crystal caves, Elbow Beach and other places all by bus. It worked out fine. The busses are clean, safe and prompt.