A cat sail is an amazing way to experience any port, if interested look at Aristocat, Kuralu, and Patouche to see if they are operating and what itineraries are offered. You can usually choose between a sail that is more snorkeling or one that is more beach bars. A great way to experience the Baths on Virgin Gorda minus the bigger crowds, or to snorkel the Indians and Norman Caves, or visit White Bay on Jost at an affordable price.

Having said that an island tour is highly recommended, Tortola is a gorgeous island with spectacular, panoramic views, beautiful beaches, and wonderful people. If you opt for an island tour contact Arnold immediately for availability as he is very popular.

[EDIT] Also, if you want to quickly learn what the most popular sights and beaches are, the Tortola forum on Cruise Critic is a great resource for day visitors, especially first-timers. Your port times may limit your options, not sure how long you have. But a tour is doable even for a short time in port.