Zurich: Skip it and go to Bern or Luzern? I'd go to Luzern myself.

But, if you've a reason to hang around Zürich. Take the train up the Uetliberg if it's a nice day. Head back down and over to the lake and enjoy a trip on one of the lake ferries (the steamships are nicer in Luzern, btw. ). Hang out in the old town. Visit the Landesmuseum. Wander down Bahnhofstrasse and look at the watches. Go to Sprüngli and have a great cup of coffee/tea and an amazing dessert (they also have a nice coffee shop in the airport). Have a meal at the Zeughauskeller if you are looking for something on the more affordable side.