St. John's is the main port and capital city of Antigua. There is a lot of shopping, dining, and historical attractions to see, and it is certainly worth spending a day walking around. The cathedral is beautiful and dominates the landscape. The Museum of Antigua and Barbuda has good exhibitions on the early history of the island.
For those looking for a little local flavor, check out the farmer's market on Friday and Saturday morning, with lots of fresh fruits and even some crafts. This is my favorite thing to go see in St. John's.