Anneau du Rhin (http://www.anneau-du-rhin.com) is the track closest to Zürich I know. It is north of Basel and it takes about 1h30min to get there from Zürich. One of the companies offering track days at Anneau du Rhin is the Cornu Master School (http://www.cornu-moto.ch - sorry, in French and German only).
Some of the motorcycle dealers in Zürich are offering track days as well. Ernes Euromotos (http://www.ernes.ch/events.htm) has one scheduled for 10th - 12th August in Mugello.