Oooh, I have lots of advice! When I lived in Rhode Island, we had family over from the UK nearly every autumn and I was the designated tour planner/director/driver/etc.
Salem is nice. I'm not really into all the witch stuff, but it's definitely worth seeing on Halloween if you're going to be there anyway. If you're going specifically to see the fall foliage, however, Halloween will be too late. Peak time is about the middle of October.
Salem also has the
House of the Seven Gables, which I love.
If you can possibly head out to western Massachusetts, there's a lot of great countryside to see there. If not, there's plenty to see in the Boston area, too! But I am partial to the country.
We always headed west to the Berkshires and stayed in Stockbridge at the
Red Lion Inn. It's the quintessential New England town! Stockbridge has the Norman Rockwell Museum, which used to be in a gorgeous old building right in the centre of town, but I believe it's moved out now to bigger facilities - that probably means there's more to see, but the old place was so nice.
Stockbridge is also near Lenox, where
The Mount, Edith Wharton's home is. That's one of my very favourite spots in New England!
There's also
Hancock Village, a Shaker village, about 45 minutes away. That's a really fascinating place!
I have lots more ideas if you want them!