what to do with 8 hours in st. thomas

[hawks56]

St Thomas is my 2nd stop on a mid April carnival sunshine cruise from 12noon-8pm and I am just curious what the best stuff to do is...I will also be in grand turk, st Maarten, and Tortola, so I would like to do something unique in st thomas that I wouldn't be able to do in the other 3 ports...I was debating between coral world to do the sea trek as well as the shark/sea lion/ turtle encounter, as well as spend the rest of the time at one of the world famous st.Thomas/st john beaches...would time best be served just staying at coki beach, or would a venture to trunk beach at st. John or Magens bay be better and more worth it? I heard Magens bay can be extremely crowded, however, I believe there will only be 2 other cruise ships in port so I don't know how bad it would be...also, I don't know if 8 hours, all of which are after 12noon would be enough time for the long travel times to st. John...I will be traveling with my girlfriend, and we are both in our mid 20s, so we are pretty much down for anything...any advice would greatly be appreciated!!