If you want warm weather I would do Florida or California probably.
Florida has almost perfect weather in March. Some time at the beach, maybe Orlando for a few days. People love where I live also, it’s just over an hour north of Orlando, it’s like a cruise ship on land with tons of activities. Live music, dancing, arts and crafts, lectures, all sorts of stuff. Disney has great live music too if you just stay in Orlando, we get a lot of the same bands, and of course Disney World. If you like golf, we have over 35 golf courses. Swimming pools, tennis, Bocce ball, performing arts, etc. Here is the calendar page. It’s a 55 and up community, I don’t know how old you are, but plenty of younger people are here also, including kids.
Some of the National Parks might be a good idea, if you love nature. You can search our national parks website once you narrow down weather you will be east or west or where. The Northwest corner of the country can have mild weather in March. The Northeast will be a crapshoot in March, it could be beautiful and spring-like, or snowing.
NYC is an obvious place to visit if you like big cities, but like I said the weather will be a little unpredictable.
Washington DC is our nation’s capitol city as you know, and late March early April is cherry blossom season. It’s beautiful when the trees are all in bloom. DC is about 4 hour drive or train from NYC. Charleston, SC was mentioned above, that is in-between Florida and DC. You could do a drive up the east coast and stop in various cities, but I personally think it is a lot of driving.
States like Arizona and New Mexico have desert, and varied climates depending on the elevation. It’s supposed to be beautiful, but I haven’t been there myself.
No matter what you find that interests you, look up the weather before you decide or ask fluther, if the weather matters to you.
I agree about Seattle not being a big deal. Going into Vancouver in Canada is way more fabulous, and just a couple of hours north.
California has all sorts of things to see, but I will leave that to the Cali jellies.