If You Have 1 Day
Take "Walking Tour 1: Zurich's Bahnhofstrasse" to Lake Zurich, where you can board a steamer for a 1 1/2-hour ride on the lake. Return to shore and visit either the Kunsthaus Museum or the Landesmuseum. In late afternoon take "Walking Tour 2: Zurich's Altstadt", along the famous quays of Zurich and through Old Town, where you might have a raclette dinner in an old tavern.
If You Have 2 Days
Spend the first day as suggested above. On the morning of the second day, visit Fraumünster or Grossmünster, the two most famous churches of Zurich. Enjoy lunch in a typical Zurich cafe. In the afternoon leave Zurich for Uetliberg for a panoramic view of the Alps and the city. Have a beer and listen to the oompah band at the Bierhalle Wolf in the evening.
If You Have 3 Days
Spend the first 2 days as outlined above. On the third day, see all the attractions you've missed before, including two museums: the Rietberg, with its great non-European art collection, and the Bührle, with its collection of modern art. Visit the Botanischer Garten (Botanic Garden) and the zoo in the afternoon, if time remains.
If You Have 4 Days
Spend the first 3 days as outlined above. On the morning of the fourth day, go to Winterthur, only a 25-minute ride from Zurich, to see its many attractions, including the Museum Oskar Reinhart, the Kunstmuseum, and the Schloss Kyburg.