If you will be in Japan during the period 3 - 7 February, I strongly suggest the Sapporo Snow Festival. Although it opens on Tuesday the 5th, it is much better to get there a day or two before to watch the work being done. Sapporo is an hour flight from Tokyo; but the trip is well worth it. You can always tour Tokyo but if your schedule matches the Snow Festival, it is a great chance to see it.

Marc