Take the ferry to St John and never come back!
St John is the island to see, you can hike there, take a open cab ride around the island for next to nothing, go to Trunk Bay best beach on this planet and snorkel.
Really, St John is unbelievable, I say everyone should visit here before they die. 2/3 of the island is national park, untouched pristine absolutely breathtaking.
Rent a jeep for the day and just drive around, it's something special to see, no comparison to St Thomas.
Here's some pics of our first trip.
http://www.virgin-islands-on-line.com/virgin-islands/viewtopic.php?p=226303We then got married there a few years ago. The link to the above forum is great for getting the inside scoop on both islands. Trust me on this, GO TO ST. JOHN for the whole day, St Thomas pales by a country mile in comparison :)