I highly recommend Boston. It is a beautiful city. Lots of kid friendly activities. Lots of great restaurants and shopping. You could visit lots of cute towns that would be a short drive from the city (or a train ride from Boston)
If you decide to rent a car, you could do a short drive to New Hampshire or Maine.
There are three Hyatts in the city (I recommend the downtown location, although the airport Hyatt is only a water taxi ride away from the city)