Taking 21 mo old DD to a zoo ~30-40 min away tomorrow. Zoo is small; very doable and easy for toddlers. She naps around 12:30-1pm. Most of what I heard/read is that you can definitely do the whole zoo in 2 hours at normal pace. There is additional petting zoo and playground on site.

Go at 10am when they open, pack a lunch to eat there at noon-ish, get in the car around 12:30-1 and let her sleep on the way back home.
Pro - Get there early to get parking
Con - Need to pack lunch, really only have 2 hrs there

Eat lunch at home, leave around 12:30 and have her nap on the way there, go in once she wakes up (let's say 2). Be at zoo 2-5pm
Pro - No need to pack lunch, have more time at the park
Con - When we leave might be a lot of traffic, might take a while to find parking (but she should be napping so we're not in a hurry)