My friend and I would sit for an hour or so and watch the lions. They were fun to watch. It took all of my willpower to avoid getting out of the car to go skritch them behind their ears. Well, that and I think they would have eaten me. Ah well.
I'm pretty sure there's a Darwin Award story about that very thing. Well, there was pot involved, but you get the idea. At the Detroit Zoo, the big cats either pace or just lie around, it's quite boring.