I went to the zoo this fall and really enjoyed it. The tiger enclosure, with the window where the tiger can walk right up to you, is fantastic.
I also really love the penguins. They swim right up to the glass, which seems to really thrill the kids, as well. And I swear, the penguins can see out and will play with you, chasing your hand around if you waggle your fingers at them a bit.
The zoo is a nice size--large without being totally unwalkable--and makes a good day trip. The aquarium is also really nicely done and quite enjoyable.