Is there a beach nearby (either walking or by cab) where you can rent kayaks. I'd consider the Celebrity's kayak tour but it's $70/person. That sounds a little ridiculous and I'm not sure I want to spend 3 hours following a guide.
Also, what's there to do within walking distance of the St. John/Antigua port? We're going to be there for quite a while and most excursions are only 3 or 4 hours. Not sure what to do if our excursion returns early.
We were there two years ago on Celebrity's Galaxy. Antigua quickly jumped to near the top of our all-time favorite Caribbean islands.
Did a snorkeling tour with Wadadli Cats in the morning. Snorkeling was good not great but the water was fairly rough that day. Had a ball on the boat, though. Super crew, great scenery, great beach stop and the wind made for some great sailing.
If you're into shopping, there's a small shopping area right near the pier.
The highlight of our stop there, though, had to be lunch at Hemingways. They serve undoubtedly the best grilled Caribbean lobster I've eaten anywhere in my eight trips to the Caribbean. (I search it out everywhere I go.) It's a little on the pricey side for a lunch but worth every penny. Check out: http://www.hemingwaysantigua.com