There's a great lobster place in Ogunquit, ME called the Lobster Pound and a wonderful seaside cliff walk called the Marginal Way. Plymouth harbor has the Mayflower II and Plimoth Plantation nearby is great too but it is built on a hill so the older folks may have a problem walking down to see the village and back up the hill to leave.
If you have time Cape Cod is nice to see also. To see things like the National Seashore Park and Provincetown however it is a lengthly ride. About 45 min from the Cape Cod Canal to the Seashore Park visitors center and another 1/2 hour to P-town.
Wherever you decide to visit, have a great trip and welcome to New England. :)
Oh, I meant to add - if you really want to see the Pops on 7/4 you need to get there veeerrry early. They don't let people go there as early as they used to do but people go and sit as early as possible until the show starts about 8 pm. (that's why I've never gone-years ago people camped out all night).