In addition to the standard city tours (which are quite nice, btw.), there are quite a few places that are open to visitors, though most are by appointment only, and several require minimum group sizes. I've been to several of these (and some more that are only possible if you know people working there), and found all of them really nice. Note that in several places, the tour might be in German only, so you should inquire should that be an issue for you.
Bus tour of Zurich Airport (there are additional, more behind-the-scenes-y tours if you're willing to pay for them and sign up in time).
Tour of ETH Zurich (every Tuesday; free, but only in German).
Zurich's train station seems to be a bit difficult to visit, but you can get tours of the train station of Bern, which is only 1h away by train from Zurich.
Another visit that might be interesting for you is a tour of a historic factory near Zurich.
Prime Tower, Zurich's tallest building, but you have to apply two weeks in advance, and they'll charge you for 14 persons even if your group's smaller than that.
NZZ newspaper printing facility in Schlieren - no mention of minimum group sizes.
Hydro-electric power plant in Zuerich Hoengg, a recently modernized power plant inside the city (every Wednesday and Friday afternoon).
Places with requirements you don't meet (but may be interesting for others)
Letter-sorting facility Zuerich-Muelligen of the Swiss postal services require at least 10 people.
Zurich lake police require at least 10 people (some of?) whom must be residents of the canton of Zurich.