The "ISG" is one of Natalie's favorite museums in Boston. It's actually an old mansion that the original owner built so that it could be turned into a museum upon her death. Her only stipulation was that every single piece of art must remain exactly where she placed it -- a requirement that was complicated by a robbery in 1990. The thieves cut some of the artwork they stole out of the frames, and the curators returned the empty frames to their original locations, in order to adhere to Isabella's wishes as closely as possible under the circumstances.
The MFA has all the John Singer Sargent portraits you need to feel like you're stepping into 19th century high society.
The Freedom Trail and Commonwealth Avenue between Arlington St. and Mass. Ave. make for great strolling with a hot chocolate from L. A. Burdick's on Clarendon St.
Certain streets in downtown Boston take great pride in going all out for Halloween, starting as early as 5pm. We love walking around the city on Halloween. Union Park St in the South End, all of Beacon Hill, and Marlborough St in Back Bay are excellent nearby options.
Salem is a madhouse in the fall, but if you are looking for a big Halloween celebration, this is it! Don't try to drive -- you will not find parking. There is a convenient rail option from Boston's North Station directly to walkable downtown Salem on the Rockport/Newburyport line, as well as a ferry, though the ferry's times are more limited https://www.mbta.com/destinations/salem. The Peabody Essex Museum in Salem has a very cool exhibit that covers the history of the Salem Witch Trials.
Road tripping around New England is a lot of fun and Boston is a great launch point, so you might as well do it while you are here! We would strongly recommend coming the week before the wedding rather than the week after for better weather and to catch the tail end of foliage up north. Some destinations and their one way travel time in order of recommendation:
White Mountains of New Hampshire - we have spent many a fun weekend hiking and skiing up here. Littleton, NH is our favorite town to stay in, but Lincoln, North Conway, and anywhere in Lake Country are good options too. Franconia Ridge is the single best day hike in New England, especially in the fall, but it is very strenuous. Text us for more recommendations!
New England's ultimate foodie town on the coast, just a short drive (or train if you're ambitious) from Boston. Bissell Bros brewery, Tandem Coffee, and Holy Donut are musts, otherwise you can't go wrong with any of the restaurants. Central Provisions and Duckfat are classics. Maine also has better lobster than Boston.
Acadia is one of our favorite national parks! Things will be a little slow in the late fall, but the park is better without the crowds - just bring layers! We usually go on 4-day weekends to make the drive worth it, but 3 days is enough, and 2 could even work if you're not a big hiker. Stay in the very cute town of Bar Harbor that is essentially in the park, and try to make it to Cadillac Mountain for sunrise (requires reservations now)
Here in New England, they say you're either a mountains or a beach person, and we are mountains people, so we've never been to the Cape in the fall. But if you'd like to check out the Cape and Islands, go for it! It is definitely low season, which means fewer hotels and restaurants will be open, but you will avoid the usual crowds and should still have moderate temperatures and great foliage.
It may be a bit late in the season but worth looking into. Many options to the north and west of Boston. We like Tougas Family Farm: it hits the trifecta of good apples, cider, and donuts. Very family-friendly too!