Travelling hokkaido-tokyo

Hi,
In your case, using a rail pass is simply not practical or economical. It would take you about 9½ hours to go by train from Sapporo to Tokyo, completely killing the day for you - and with just a week, you don't have much time. It would be far more sensible to fly (in fact, if you run the route on Hyperdia, it would put you on a plane). You can get a discount air ticket such as on Jet Star or Peach for a fraction of the cost (as low as ¥4430), and you'll be in Tokyo in just over 90 minutes.

In Tokyo a few of the best places to see are the Tsukiji Fish Market, plus the site that every last tourist goes to see, the Sensoji Temple. If you are in Tokyo on a Sunday afternoon, be sure to go visit Harajuku to see the youth with their wild fashions. Meiji Shrine and Yoyogi Park are next to it and well worth seeing also. And be sure not to miss the Shibuya Hachiko intersection especially on a weekend when it looks like half the city is crossing the street all at once.