Zoo Lights in December is a definite must do if you have kids. But even if you don't, it's a great place to snuggle up with a date, walk around with a beer and enjoy the thousands of lights hung throughout the walkways.
Besides that visit, I've also been in June when I've gotten to see the giraffe habitat and meeting areas. There's also a banquet hall on-site too.
The zoo itself is decent size. It's not TOO big and expensive like the San Diego Zoo where you have to budget your time and dollars wisely. But it's also not a tiny zoo where you finish seeing it in an hour. At $14 adult tickets are pretty reasonable.
They have all kinds of themed days/nights with fun programming for kids. It was even recently voted as one of the top 5 zoos in the U.S. for kids.