A Bach fugue. Or Berg's sonata. What have you been playing recently? It would be helpful to know what your "level" is.
EDIT
Excuse my impertinence; I looked at your posts to find the pieces you play. Anyway, Bach is always good for concentration, and for one's health. Try Fugue No. 9 from the WTC Book 2, and play it at a very moderate tempo. In any case, I suggest you play some thing that isn't too fast. I myself have been studying Schoenberg's Op. 19 pieces, which requires a great deal of concentration to play.
I don't recommend playing anything too "romantic"; the sustained notes tend to make one giddy and one loses their concentration quickly.