Hi there! Looking for suggestions on stops/nice places to stay/activities along a route from Boston to Quebec City then down to Plattsburg NY. Planning to stay in NH White Moutains area one night and then need a place between there and QC. From QC whats good on the way back down to NY? Looking for fall sights, cute bed and breakfasts, beautiful drives, shorter hikes. Any suggestions welcome! Thank you!

If it's a saturday, you could take the option of going I93 to I89 to I91 and stopping at Norwich VT farmers market. It's very festive with crafts and live music. King Arthur Flour is also in Norwich and offers great shopping for baking ingredients and has a nice lunch cafe. If not a saturday and/or you aren't interested in the farmers market, you could still do the route but get off I93 and take a route that goes past Concord Hospital. This becomes a lovely drive thru the towns of Hopkinton, Contoocook and Warner (see Mt. Kearsarge info for short hikes. From the Wilmot side which would require a detour, you can drive almost to the top with nice views and then hike the rest of the way - see Winslow State Park). In Hopkinton take a short detour to Gould Hill Orchard to buy apples and try some of their hard cider (check tasting hours). View from picnic table/parking area is nice. Get on I89 at exit 9 to continue to Vermont.
Or the quickest route you probably planned on goes up thru the White Mountains. I thought the visitors center at The Flume was very interesting. Although you have to pay to hike, if you haven't done it before the Flume is great. Depending on your interests as you head north you could make some nice detours such as get off I93/exit 17 (short hike to Hannah Dustin statue?) and head to Canterbury Shaker Village. Drive past, visit the gift shop, take the tour. There's a small distillery just past the village but you'd have to check hours. Get back on I93 at exit 18. You might have to back track, I'm not sure of route between exit 17 and 18. You can get off at exit 23 Meredith to visit Hermit Woods Winery. It's a beautiful town but expect congestion on the weekend. There is a variety of hiking trails at the Rocks Estate in Bethlehem. I did a spring wildflower walk that was nice. The history made it more interesting. https://therocks.org/trail.php

No advice on coming back from Quebec except to agree with HappyTrvlr that Burlington VT would be a great choice. You can ferry across the next day. Coming south you could drive the Hero Islands and then end up in Burlington. We haven't done this in the fall just summer. Don't expect much commercial activity, it's very rural.