All of the above are great suggestions. A trip down to Rapperswil to see the Castle there might be interesting, if you've not too many castles. The Kunstmuseum (Art museum) is also pretty good. If you're out here on a Sunday, be forewarned that pretty much all the shops outside of a few railway stations, and the airport are closed.

Luzern is just a bit over an hour by train away from Zürich, and would probably be worth it if the weather is good. In the other direction, near Schaffhausen, the Rheinfall is the largest (not highest though) waterfall in Europe.