Onset is still a lovely place. There’s a shul, and accomodations are cheap. It is on the mainland side of the Cape Cod Canal, so there’s no bridge traffic to put up with. There are a few nice beaches, the water is pretty warm and still, since it faces Buzzards Bay.
One village over, in Wareham, about five miles away, you can see Cape Cod Baseball, which is extraordinarily good. The Cape Cod League is sponsored by Major League Baseball, and is an invitational league for the best college players in the country. They can play there and be scouted and NOT lose their amateur status. 38% of all current major league players played in the Cape Cod League at one time or other. I saw Nomar Garciaparra, Mark McGwire, Mo Vaughn and many others play BEFORE they hit the big time. Admission to the games are free, donation requested.
Also within a few miles – Cape Cod Canal – there’s a great bicycle path along both sides of the canal. The Army Corp of Engineers has a museum on the mainland side.
I vote for Onset, too.