Each time we've gone (once for zoo, once for night safari), we've taken taxis. It takes about 30-40 minutes from most places in town. ding-dong! ding-dong!

There is a taxi stand outside both that you can use to get back - I was originally worried about that. Especially for night safari, there are a lot of people coming and going at the same time, so it's no problem. Additionally, at least outside the zoo entrance, there is one of those little "call a cab" machines that operates for a dollar or so. I highly recommend those whenever you see a lineup of people. It issues you a ticket with a number on it, and you can bypass the line when your cab shows up. Also important at the base of the tower (middle) cable car stop.