Well here are my two cents:
1. st.Johns tour is a good choice great of beaches and also a good place just to "discover" during a cruise the views and photo oportunities are great.
2. Havensight Mall is even a good place to explore but there are a few activities walking distnace from it, there is a butterfly farm near port ($15 dollars price per person for the tour) a nice place for the kids and also there is the mountain top.
3. The last time I was there I remember there is a place near port where you can rent scotters and see the island by yourself.
so maybe you should try the st.John tour as I remember the tour boat, at the end, leaves you in front of the ship.