2 nights, 1.5 days in Switzerland

Luzern.
Take an early train to Luzern (takes 3 hours). Or you may want to spend the morning in Geneva, and go on at lunchtime.
Two nights in Luzern. Look round the town, take a boat ride on the lake. Go up Pilatus (the local mountain).
Trains from Luzern to Zürich Airport (Flughafen) run every ½ hour and take just over an hour. Leave Luzern at midday and you will have plenty of time.

Option 2 - Somewhere in one of the side valleys near Interlaken: Grindelwald, Lauterbrunnen or Mürren. But this will involve more time getting to Zürich airport on day 2 (3 hours).

This looks like a short weekender. If a scenic railway journey is to your liking, you could head to Luzern using the Golden Pass Panoramic line followed by the Interlaken-Luzern Express over the Brunig pass. This way takes twice as long as a mainline express train from Geneva to Luzern, but might be of interest. If you are traveling light with just an overnighter back pack, you could stop off in towns along the way, if you are not in a hurry, as the trains run at least once per hour in either direction in Switzerland. You have to change trains in Zweisimminen, so that might be worth a stop. Interlaken doesn't really trip my trigger as far as a stopover.

As Chris said, Luzern is a more attractive place to spend the night than Zurich. There are direct train every half hour to Zurich airport, making it almost as quick to get to the airport as in Zurich itself. You can spend the morning and early afternoon in Luzern before heading to the airport.