The Woodland Park zoo is really special. It's divided into sections according to the areas of the world that each animal lives in. For example, there's the "tundra" with brown bears, otters, elk, wolves, etc - the "safari" is where you'll find lions, tigers and wild dogs. (Nope, not hyenas, wild dogs.) and so on.
I like this zoo because it's small enough to be easily walkable for kids, yet large enough to have a wide variety of animals so you don't get bored. Often with our young kids, we don't "do" the entire zoo in a day - and for this reason, the price of an annual membership is well worth it.
The animal enclosures are realistic - I like that they don't look like cages. The zoo really makes an effort to bring to life the animals habitat as it would look in the wild.
One of my favorite exhibits is seasonal - the butterfly house is amazing. It does cost an extra dollar or two to get in, but it's well worth it. They give you a color guide with pictures of each species of butterfly. Often the butterflies will land on your shoulder!