Zurich Airport

Only if you enjoy going through airport security again.

Possibly. Train into city about 20 minutes. As previous poster mentioned you will have to clear immigration and customs first and then go back and do security again so you should factor in the timing. Might not be worth for 5 hours.

That is about it --- to town center, Turn around and come back. Unfortunately I think the five, sometime six hour, layovers are a sucker play. You think you have enough time to do something but you really don't. O' you might be able to walk around Zurich for an hour or so but not worth the hassle of the transportation in and out, immigration, security, etc. Find a nice corner to relax in, maybe a quick nap, and a decent lunch somewhere.

5 hours is short, but Zürich is probably one of the few places where you could go to the city without too much risk. With good timing and immigration luck (typically OK in Zürich), you can be in the city 1 hour, perhaps 1h30 after you land. This gives you 1.5-2 hours to play with; you then need to be on the train to the airport no less than 2 hours before your Venice flight (since it is advised to arrive at the airport 90 minutes before intra-Europe flights).
Now, would I do it personally? Probably not. With 6 hours yes, not with 5.