Day Trip from Boston to Maine
It would help to have a bit more idea what you're interested in seeing and getting a feel for in Maine. For instance, four immediate thoughts for southern Maine on a day-trip or short overnighter, but only one, or maybe all, will be what you're looking for!
If you are an outlet shopper, there are big clusters of outlets in Kittery, just over the border from Portsmouth, and at Freeport (north of Portland), which is also home to LLBean.
If you're in to wacky-tacky fun with a nostalgic feel, the amusements pier and beaches at Old Orchard Beach (south of Portland) are open through mid-October (although I wouldn't count on go-in-the-water temperature
At Wells, a bit further south than OOB, you'll find the Rachel Carson National Wildlife Refuge, which has wonderful walks through extensive wooded areas and down into tidal salt marsh (on a dry boardwalk). Depending on day or time, you'll either be on well-explained self-guided walks, or with a ranger.
If food is your thing, Portland is your place. Over recent years it's become a tremendous foodie scene, with lots of good food and bread. Don't go only for the big gourmet deals, though. Two of our favorites are Becky's Diner on the Commercial Street waterfront (it's definitely a diner, but much more; really good dinners and it serves as a community meeting place) and the Lobster Shack, south of Portland at Cape Elizabeth. Reasonably priced, fresh lobster, lobster rolls and more, in a simple room or out on a terrace right on the beach.
More details obviously on anything that interests you...