We lived in St. Louis (pre-kids) and were about 5 minutes from the zoo. I really liked hitting the zoo and just perusing everything it had to offer. Now I live in Seattle and have two bumpkins who generally go stir-crazy unless they can get outside. We hit up our local zoo all the time, but it ain't the same as the St. Louis Zoo.
Many, many living creatures to check out; a seriously fun carousel; lots of things to do and ride on, way too much bad stuff to eat. It's everything you want for a day of fun.
Best part -- it's free. Not so in Seattle, where you have to pony up $$$ to go and/or join. Easily a worthwhile destination while in Saint-Lo-Mo.