Eastern North America:
NYC
Chicago
Montreal
Quebec City
DC (but it will be hot)
Other things my boys would have liked at that age:
Dude Ranch vacation
Family Camp - we did one in Sequoia Nat'l Park years ago but they are all over the country. It was awesome, lots of activities and can be as structured or loose as you want.
We haven't been to Iceland yet but it's on my list. Everyone we know says it's perfect for a shorter international trip since you can do lots in less than a week. But it's not inexpensive. London or Paris and their surrounding areas would be very doable in a week trip and easy to find direct flights out of Boston. I've traveled internationally twice with just DS1 over the years. DS2 and I are planning a trip together for summer 2026 so I don't think it's crazy to go out of the country with one kid.
Mom to Two Wild and Crazy Boys and One Sweet Baby Girl