Columbus Zoo and Aquarium
A friend recommended getting a family pass to the Columbus Zoo and Aquarium, and I'm so glad we did. Between entry and parking fees, it paid for itself in three visits, plus if you are planning to travel, you'll get free or reduced admission to over 100 other zoos around the country.
It's a really wonderful zoo; quite large and pretty. The petting zoo is great - the animals are clean and mellow and well cared for. And the gorillas are always a huge hit. Every October they put on Boo at the Zoo, which is a lot of fun, and in the winter it's Wildlights - the whole zoo is lit up and twinkling, and kids can go ice skating or check out the model train display.
One caveat: if you're going mid-day, I recommend packing a lunch and taking advantage of the picnic tables in the food court area, because the lines to purchase food can be quite long.