I lived up there a couple years. I went to Portland, ME a couple times but unfortunately didn’t get to explore ME outside of that or RI at all.
I know NH and VT though. Portsmouth, NH is great. On the water with several cool restaurants and bars. Mansions in Rye Beach, NH are cool to see. Second the Mt. Washington comment. Weather can be crazy on the mountain itself, but the hotel is cool to see. Breton Woods conference was there, and Thomas Edison did the lighting if I remember right.
People talk about the leaves in the NE, but I love NH/VT in late May/June. Never seen something so green. Woodstock Inn in Woodstock, VT was a nice place to see and has drinks and a game room downstairs. I’m a big fan of Burlington. It’s on the lake, and you can probably track down some Heady Topper, which was Beer Advocate’s #1 beer for a long time. Shelburne Farms is nearby. It was this farm owned by the Vanderbilt’s (I think) and you can walk around on it, hang out, etc. You have to have a reason to be there, but I just went for breakfast then stayed a few hours
Edit: wanted to add that Whistlepig looks like they have a tasting room in Queechee now. That’s right by Woodstock. Don’t think that existed while I was living there, but that’s where I first got into Whistlepig. It’s in the same building as Simon Pearce, and you can have lunch/dinner there over rapids/a waterfall