If you plan to offer sandwiches and/or seafood I would suggest offering lobster rolls with choice of butter, plain, or mayo lobster filling with options to add celery, cucumbers (diced like at Summer Shack!), lettuce, etc.
If you want to go the seafood route, Summer Shack seems to have a good model because it is always busy whenever I've been there!
http://www.summershackrestaurant.com...enu%202010.pdf