Warszawa Zoo
Zamosc zoo is far better, but it's not really a day trip (four hours on the train each way). But the town is quite possibly the nicest in Poland, so it's well worth a night there.
And you can lick the walls.
Normally I recommend that people who are going to visit take a small bottle of tequila and a couple of limes, but I doubt that would be appropriate in this case.