Others have mentioned this, but taking the train is a great way to make travel part of the fun. The train is much more comfortable than driving or flying, and if they’ve never rode one it’ll be a cool experience. I saw you’re sticking to the east coast which is great for train trips. If you’re in the Northeast then the Vermonter is a beautiful ride that goes from DC to St. Albans, Vermont, right on the border with Canada, also making stops in Vermont’s major cities and a bushel of other states. You’ve also got the Downeaster between Boston, MA and Brunswick, ME which is very pretty in the winter if you like the cold. If you’re closer to the south then the Palmetto (starting in NYC) has service between DC, Raleigh NC, Charleston SC, Savannah GA, and Jacksonville, Orlando, and Miami FL (last stop). Finally the Northeast Regional will get you between DC and Boston with any major city you could want, but it’s a big commuter train and not as pretty for the kids. Something like the Vermonter would have a very “we’re taking the train to a winter wonderland” aesthetic, but if your kids like trains/cities then the NE regional is cool because the whole ride is “Look at those trains! Look at that city! So much industry!”

I’ve loved hiking since I was a kid. I wouldn’t go for any long hikes lest they get tired and bored, but who doesn’t like spending some time outside?

Aquariums and natural history museums can be a blast. If any of your kids have a specific interest in something (history, art, aviation, etc) then there may be other museums they’d like too

Unfamiliar transportation can be a freebie between activities - a boat, a train, a cable car, even a double-decker bus hit different than a car ride

Obvs, having some extra activities that you won’t mind canceling can help leave time for gratifying any whims that strike the vacationers, without putting pressure on them to find something that strikes them

Landmarks can be cool, but crowded ones usually weren’t worth it to me as a kid. But then, maybe your kids don’t mind crowds

Swimming. Just swimming. If it’s somewhere cool, then cool. If it’s in a motel pool, then that can still be cool

Depending on where you are, I recommend a small mountain to tech them skiing/snowboarding and leave the possibility of one of them just going tubing all day. Personal preference of course :)

Children’s science museums are cool, not sure how common they are but I’m aware of at least two cities that have one in the Midwest-region so they can’t be that rare.