St. Thomas, more so than Tortola, is easy to do on your own. There is also a butterfly garden that's walking distance from the pier, and the skyride. The night before is usually Pirate Night, so that can be a later evening for many, so even if you do a chill Tortola outing, being up bright and early for a St. Thomas outing may not be everyone's cuppa.
We're doing the Eastern next year, and our plan is big Tortola day, fireworks, zonk, Cabanas brekkie then do a cab to Magen's Bay on our own time and terms.