At the farthest southeast end of Trellis Bay, in the vicinity of the Beef Island Guest House & de Loose Mongoose, there's a good snorkeling spot with patches of coral and a lot of seagrass. There' s little or no boat traffic here, making it much safer for snorkelers. Sea tortoises and rays can be seen, amongst other critters that inhabit the sea grass areas.
You can take lessons in windsurfing at Trellis Bay, with Boardsailing BVI. I watched this novice windsurfer for almost an hour while awaiting the Northsound Ferry to Virgin Gorda; it's not that easy to get the sail up & get going! If there's not enough wind, you end up falling over again. If a gust hits you before you're braced for it, down you go again. But he was thoroughly enjoying himself in that lovely warm Caribbean water, so I think it was well worth the effort!
Boardsailing BVI also teaches kitesurfing, and rents equipment for windsurfing, kayaking & kiteboarding.
Trellis Bay is fun for people-watching, windsurfer watching, and just absorbing a bit of the local color. Artists have their studios strung along the bay, with various fun little cafes that feature local foods in between (and a cyber cafe for Internet & VT junkies). See Shopping Tips & Restaurant Tips for more details.
Tortola has quite a reputation for the festivities that take place during a full moon and the reputation revolves around partying and mushroom tea.
For something a little more, dare I say...mature...investigate Aragorn's studio during a full moon.
I have been to the studio and found it an extraordinary experience. The metal scupture is fab. It's yet on my list of things to do to revisit during full moon.
The giant metal orb you see on the website is set alight on the water during the festival. Must be quite a sight.