You've picked a nice variety of locations.
My first thought is that the climate in each location is going to be pretty different that time of year. You'll want a winter coat, gloves, a hat, and boots in New York, Chicago, Pittsburgh, Washington DC, Boston, and Niagara Falls. You could probably get away with a t-shirt and light jacket in New Orleans, Orlando, and Charleston. It just depends on your temperature comfort level. I think anything below 0 degrees Celsius is cold, but there are others who think 15C is cold.
People might say you're crazy to go to Niagara Falls in the winter, but I think it's quite lovely in the winter. Plus, you'll have fewer crowds to deal with. It might be too cold to enjoy a ride on "Maid of the Mist", though, and I might not hang about the tunnels and caves where you can feel the spray. I'd stay up top where you can see the falls going over the edge and then maybe walk along the river and check out the rapids.
Fat Tuesday (Mardi Gras) is March 5, 2019, so New Orleans will be ramping up for their huge Mardi Gras celebration. I know the parades usually start a couple weeks beforehand. Saxman would know more, as he lives there. Maybe he'll chime in. If you aren't into crowds, you can explore the graveyards, Garden District, streetcars, and voodoo shops. New Orleans is a beautiful city with a great vibe. I've never been anywhere like it. Do be sure to pick up some Mardi Gras beads, though. They're the quintessential New Orleans souvenir.
What type of things do you want to do in the other cities? I can give you lots of tips regarding Chicago (I live a couple hours away and go there all the time).