St Thomas is one of the larger US Virgin Islands. If you are visiting for more than a couple of days, it is a great idea to rent a vehicle and take a self-guided tour around the island - as long as you remember that they drive on the left side of the road. The vistas are magnificent!
Driving from the St Thomas airport (on the west end of the island) to the Red Hook barge (on the east end of the island) can be an adventure!
There are some pretty wonderful beaches on St Thomas; this map shows where they are located (thanks for Frommer's):