the town of greenport is probably the only town on the north fork that offers alot of shopping in a small walking area...and there are alot of art galleries there as well. I'd recommend the Frisky Oyster for your dining pleasure...the food there is spectacular.
most of the beaches require parking permits, and it hink you can get a day pass. there is on on Gull Pond in Greenport that has a launching ramp as well. It is a bay beach that is just to the east of Claudio's so you can see alot of boat traffic if that's want you're into
as far as beaches on the sound, get on the north road(48 I beleive) and head west. There a couple of nice beaches on that road and not too far from Greenport.
The only other real attraction on the North Fork are the wineries...if you're into that sort of thing, they do wine tastings all day long...there are several wineries on 48 as well if you go further west.
New Suffolk is a nice little spot with beach, and Legend's resturant and sports bar. it's a tiny little "town" which is jsut to the north of Robin's Island. It's very quaint and not too crowded. the beach is just south of Legend's...
give me a call if you need directions to any of these spots, or you can mapquest it...they're pretty easy to get to and not too far from the ferry.