It is so easy to take the ferry to St. Barts. It is inexpensive and a fun days activity. The ride takes about an hour and a half unless one of the engines quits and then it will take you three hours to get back! But you are on vacation anyway so what is a little delay?
Once on the island the best thing to do is introduce yourself to another couple, get a taxi together and tour the island. The driver will take you to all the highlights which include the houses of the rich and famous and a few beaches before he will drop you at a beach to eat and swim for a few hours. Bring water with you! It is five dollars a bottle on this island!
After this, you will hve just enough time for a quick tour of the harbor by foot stopping in a shop or bakery for a moment or two before reboarding your ferry for St. Martin.