Chatham (Particularly Chatham Bars Inn) is very romantic area, w/out being overcrowded. It is expensive, but if it's romance you want, that's my first choice.
Hyannis is a zoo in the summer, and is a hotspot for night life, bar, clubs, etc, abound in the downtown area. Traffic is a nightmare though.
Falmouth is fun - great beaches, some fun pubs and great restaurants in town.
But if I had my choice, overall, I'd go to Martha's Vineyard and stay in Edgartown. There are plenty of Inns to choose from throughout the island. Instead of paying big $$ to bring your car on the ferry, consider rentng a car there, or if you are adventurous, mopeds are another option.