Fastnacht is terrific but a completely different atmosphere to normal workaday Luzern. Both are worth the trip. For a standard non-Fastnacht day out, try the Rigi-Vitznau-Luzern trip mentioned on this thread. (best on a clear day)

If you're coming to Fastnacht and there's any way you can stay out all night, I highly recommend doing so. Trains start running again at 5am, so you can still get back to ZH, have a shower and divest yourself of confetti before heading in to work.