I thought about recommending Chicago - certainly lots to do that is family friendly, but the weather is inclined to be dicey in March. Same with Boston. In fact, I'd prefer Boston to Chicago as far as stuff to do. Have you been to Montreal? We live in Buffalo and have a lot of friends that like to make the drive there. I'm not sure what there is to do, but we have friends with families that have gone for long weekends and really enjoyed it.
Another US city I love is Atlanta. CNN headquarters tour, huge aquarium and zoo, Coca Cola headquarters tour, amazing dining, usually an easy flight. Just throwing it out there!