There is a mall adjacent to the Airport. I haven't been there for years but it should use up some time. In the bottom of the mall is a train station. It's not a long ride into Zurich, but probably you won't have enough time. You could always go to one of the neigbhoring villages (say go one stop on the train) and wander around.
Do not underestimate your time at the airport. It took us two and a half hours to get through their double security system in June. First, you wait to go through regular security (xray, etc) Then you are sent to answer questions about your travel. Passport check, etc. Not a hassle, pretty smart in reality, but it was a tedious process. I don't think I would leave the airport at all. There were some great looking shops and plenty of cafes and food places that we saw. Unfortunately, we didn't get time for a meal because the security took so long. If you took a train or cab out, you would face this coming back.
What time of day?
My wife and I just returned from Zurich. Forget a cab; you can go straight to the train station underneath the airport and be in downtown Zurich (the main station - Hopfbahnhof (sp?)) in less than 15 minutes - it's ridiculously convenient, and MUCH cheaper than cabfare. I believe the train ticket was approximately $5-6 USD one way.
When we were there we did do at least one stereotypical visitor thing - coffees and pastries at Spruengli, on the Bahnhofstrasse about a 5-10 minute stroll from the train station. It was wonderful......and it was totally full of locals. Wouldn't mind being there right now, sitting outdoors enjoying the day and their delectables.......