We enjoyed Bermuda when we went there, although from DC I'd probably just do that one as a long weekend sometime. You can do the whole island in about 3 days. Also, in March it'll still be a bit on the chilly side there, being about 1,000 miles north of your typical Caribbean island.

March is among our favorite months for European city trips as well. (Rural trips are a mixed bag because some things close in the offseason.) Should be good fares and lots of nonstop Star Alliance destinations to pick from. Imagine Paris with no lines at the museums and lots of excellent restaurant availability... The weather won't be any crappier than a winter day in DC.

We spent a week on San Pedro island off the coast of Belize in 2006. That was a great location for snorkel/dive excursions. Also, there were boat operators offering daytrips into the mainland jungle rivers, Mayan ruins, zipline areas, etc. At the time, it was all mom & pop type resorts on the island (not sure if that's still true), so not likely places selling package deals to college students. You could do a few days out there...maybe then a few days in Yucatan. We flew into Belize City on AA and took a Cessna from there out to the island. Funky little town on the island with some interesting restaurants and a modest amount of nightlife. We felt safe walking everywhere at night - a friendly town.

Asia seems far for such a short trip. Europe and Central America are definitely doable...maybe even South America if you have a good routing out of IAD.