Hi simpson217, welcome to Cruise-Chat. Congratulations on your up-coming cruise. You really are generous. One thing you need to check into, is your age requirements. It used to be 25, but not sure if that's for all cruise lines.
Since you asked about Antigua, I'll tell you what we did the two times we were there. First time we went on a catamaran sail/snorkel. There were so many jellyfish in the water you couldn't enjoy the snorkel. This was late October/early November. The beach they took us to next was a nice beach and no jellyfish, so maybe it was just where we were on our snorkel. My hubby drank too much rum punch, so we didn't get to do anything after the snorkel except go back to the ship. The last time we were there, we walked outside of the terminal and found a local that took us on a tour for $25/pp. We went to Shirley Heights then over to English Harbour and Nelson's Dockyard. He then took us to a nice beach for a few hours. Much better experience. Not sure what other ports your visiting, but hope this helps a little on Antigua. Best wishes!