I live here in the US, and have been to all of the cities discussed up to this point. I would tend to avoid Boston, New York, Chicago, and Niagara Falls in early March. Not because there is anything wrong with the cities that time of year, but more so because of the potential problems with travel schedules and connections.

Washington DC; Nashville, Tennessee; Atlanta, Georgia; would all be cool not cold locations with plenty of site seeing opportunities, and great food tasting opportunities. Some of the regional cuisine is absolutely wonderful and should be sought out in every location. These cities are all less that a 2.5 hour flight from MCO, and you will find excellent accomodation rates at that time of the year.

Branson, Missouri is currently one of the travel hot spots in the US, it is an entire town dedicated to musical stage presentations and loads of other diversions. Here is the website for the City of Branson http://www.branson.com/ Early March weather in Branson will average highs of 7°C and lows in the 0° C range.

Some of the truly beautiful areas of this country are indeed winter delights, but unless you are ready for temperatures in the -6°C range, it is best to stay a bit more to the south.