If you haven't been to the St. Louis Zoo, you're missing out on a great FREE activity. If you're not a member, you can park for free on the streets around the Zoo (get there early though!). And if you come on a Friday evening you can see the animals and enjoy a free concert from 5pm - 8pm (concert series ends 8/30/13).
I found these cute Zoo Passports on the Disney Family website.
The PDF file (link above) includes stamps that you can put on the pages when you spot the animals. I didn't have sticker paper to print on so I bought star stickers instead. I double-stick taped them to the back of the book so that they wouldn't get lost as we roamed the Zoo.
The girls had fun scouring the Zoo for all the animals and filling up their Passport with stickers.
Also, bring a few crayons and the kids can color in the animals in their Passport book during lunch.