Here are some places I'd suggest you consider. Not all are "day stops," some are quaint little stops along the way:
Salem, MA- This is very touristy now, with all the witch-hype, but there are some lovely old buildings along Derby St, there's a great shopping area (with tourist prices), a breakwater you can walk out on, House of Seven Gables, and Ye Olde Candy Comapanie, which is the oldest, continuously operating candy shop in the country (be prepared- great chocolates!).
Gloucester and Rockport, MA- Basically loop through Cape Ann. I always love driving the back streets in Gloucester, going out to Bear Neck, etc.
Newburyport, MA
Portsmouth, NH (definitely poke around Strawbery Banke, it was a great experience)
York, ME. If the Village area is open you can tour some old buildings- schoolhouse, jail (gaol), cemetery. Otherwise drive up along the coast from Portsmouth through Kittery and along 1A- goes right along the beach- and out to Nubble Light, then back into downtown York Beach for some great salt water taffy at The Goldenrod. Then follow Shore Road up to Ogunquit.
Ogunquit, ME
Portland, ME- Very fun up-and-coming city with great food, drink, shops. Old Port area has pretty pricey hotels, stay in S.Portland or out near the airport, and NOT on the weekends in the summer!
Boothbay Harbor, ME
Camden, ME- Lovely seaside town, with a pretty nice state park (Camden Hills, Mt. Battie) with non-challenging hiking trails.
Bar Harbor, ME- Not always my favorite (VERY busy and touristy) but Acadia National Park is here if you want to hike or just drive park loop road and see some amazing views and stopping points. Downtown Bar Harbor is touristy but fun, and you can do whale watches, puffin watches, sail on a schooner, go lobstering, etc. This was all pre-covid, I'm not sure how they are handling tours now.
SO that's just an idea. Length of time depends on how long you want to be in each place, how tired you get poking through shops, how you want to spend your time, and other places you want to see. I'd avoid Cape Cod in the summertime like the plague (ok, maybe that's not a funny reference anymore, but you know what I mean!)