It is a very small island, very easy to get around. When you dock, just wander around Oak Bluffs for awhile. IF you want to go to the beach, hire a cab to take you to Gay Head. If you want to sightsee the island, hire a cab for a few hours, have a nice seafood lunch on shore.
The buses are great. Small island, lots to eat there, good food, nice clothing though on the expensive side. By riding the bus you get to see the whole island. Depending on how long you are there makes a big difference.