Pfaffikersee and Grieffensee both frose over in Feb this year and there was skating on both for a week or so, I was told the last time Pfaffikersee froze over was 4 years ago and the last time grieffensee froze was 14 years ago, so not sure if you'll find a frozen lake next week?

There is a skating rink at landesmuseum at the moment which is free

http://www.liveonice.ch/zuerich_index.html

and of course Dolder as someone previously mentioned

Nicky