If you ever will be in Tokyo for more than a few days, we can highly recommend a day trip to Lake Ashi and the town Hakone-Machi. Ideally, you really want to stay overnight. The reason you want to do this is the amazing view of Mount Fuji seen in the distance over the lake. Hakone is home to several traditional spas, so you could easily combine your trip with a visit to one of those. We actually stayed at a hotel right by the lake and had a wonderful room with lake view. I still can't figure out why the pictures we took of Mount Fuji did not work out but we have the memories in our head and that's the most important thing anyway. There is lots to do and see in the area, I promise you won't be bored.