I vote for Hakone or Nikko. Personally I just love Hakone. Kamakura would be the 3rd option. It depends on what you're into. Nikko could be pretty chilly in March.
As ksandness says, having a Jpn Rail Pass will open up even more options for you. There're many places within 2hr radius of Tokyo by bullet train... although Hakone and Nikko would be right up there with the best of them.