Italy, north or south is just spectacular in May.  And you will be a little ahead of most of the other tourists.  Last year went small town hopping in Umbria - wow, going back for more.  Don't forget to spend some time in Urbino.  Sicily just as good. 

This year going to Amsterdam in May.  Can't decide if side trip should be to Maastricht or Bruges.  Anyone been to both?