I had the same reaction at first regarding your budget. 30 euros per day won't work. 50
could work if you more or less stick to self service restaurants or self catering, hiking instead of cable cars, visiting inexpensive sights and confining alcohol consumption to bottles bought in the supermarket.
That said, here are some tips and recs:
An expense that can really blow your budget are those damn cable cars (short to medium ascents can cost 6 to 15 euros!) Hike, hike, hike up and down those mountains!
cheap accommodation in zurich: city backpacker. www.city-backpacker.ch
downtown and convenient
cheap food in Zurich: East Bank area: cafe zähringer on spitalgasse. an alternative cafe run by a collective. live music at night once a week. Crazy Cow on Leonhardstrasse 1 quirky! swiss food!
to do: stroll around the shores of lake Zurich, do a walking tour of the old town, visit the Lindt chocolate factory
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EDIT:
here's what I wrote on another thread...
I really love the small towns/villages around Lake Lucerne- especially Vitznau and Weggis.
Here is a good site
http://www.the-best-of-lake-lucerne.ch/en/welcome.cfm?
that lists sport offers and events.