Any suggestions for the area between Cape Cod and Quincy?
This is a tough one, at least for me - the South Shore doesn't get nearly as much attention as elsewhere in Eastern Mass. I presume you are looking for something more-or-less directly en route? Then a few suggestions:
The Hingham Lobster Pound, on Rt. 3A in Hingham, is a popular seafood destination and generally quite good. I've had limited experience with this place, but it has a definite following. All the usual fare you'd expect of a clam shack is available.
Along the historic waterfront in Plymouth, right by (the overpriced) Plymouth Plantations, there are a number of small establishments serving ice cream, fried seafood, sandwiches, etc. I can't say any of them in particular stick in my mind, though everything I can recall getting along this stretch has been acceptable at least. Mostly it's just a real nice place to have a snack and wander around a bit if the weather is good, see the Rock and so forth. Someone with greater knowledge perhaps could be more specific in recomendations. One thing I do like to pick up here for later consumption is cranberry curd, which is available in a number of the shops.
My girlfriend, who lived in Quincy through her later teens and whose parents are still there, swears by La Paloma, a Mexican restaurant on Newport Ave. I haven't been. The Clam Box, right across from Wollaston Beach in Quincy, is a fine place for a local experience: get your food - typical fried or baked seafood - and eat outside or take it to the wall along the beach with all the other people in Quincy. It's not exactly Kelly's at Revere Beach, but it's still fun.
I assume that if you wanted to go into Boston you would have specified, but one suggestion: just across the Boston line on Dorchester Ave. in Dorchester Lower Mills is a very good ice cream shop, The Ice Cream Smith. Well worth it.
If you are venturing further afield than Rt. 3 to Boston, Fall River and New Bedford have Portuguese food worth looking into. Also in New Bedford, Naughty Dawgs is a fun hot dog joint. A wide array of different preparations and styles of dogs available (including, if memory serves, a New York System under another name.) It's located in the waterfront/National Park district, I believe across from the large Compass Bank (sorry, don't remember the street.) I spent a couple of weeks working in New Bedford and it seemed like everyone working in downtown (admittedly, not all that many people!) went there for lunch.
Somewhere around New Bedford there's a great local soda shop, but I've lost the name - for another post, I guess. Hope this helps some.