Bridgemans on Nantasket Beach (Hull) is probably about 15 minutes away and never disappoints. http://www.bridgemansrestaurant.com/
MUCH more casual near there in Hull is Jake's (glorified clam shack but very tasty and really friendly service).
I love the Square Cafe in Hingham Center (less than 10 minutes away from Music Circus) but no idea if they are open Sundays. http://www.thesquarecafe.com/main.shtml
Atlantica in Cohasset, right on the harbor, has gorgeous views and a Sunday brunch (until mid-June anyway) but I have never really heard anything enticing about the food so I'm not really recommending it.
Several other nearby places don't seem to be open for Sunday brunch/lunch.
Second Atlantica in Cohasset. Took family gathering of ten there three months ago. Was wary because had heard that prices were in the stratosphere but very little word on quality of food. Was pleasantly surprised. Had excellent evening. All meals were excellent. Far above local expectations at reasonable prices. If you are looking for a nice meal this is a good spot. Suggest reservations with request for harbor-window table. Enjoy!!!