Once you have decided where you are going, and you know what goals you have set for your learning while you are there, you need to decide how to meet those goals.
If you are going to the zoo to study wildlife native to Africa, you will want to get on the zoo website and find out what animals they have from Africa. Then consider how you can travel through the zoo to see the animals you want to see while meeting your needs. One concern we have with toddlers is making sure that we can pass a restroom every 15-20 minutes, take time to think about what concerns your family has and how you can meet them during your trip.
If your zoo has signs with loads of animal information and maps, you can plan to spend time at each animal enclosure reading them and checking out the maps. Try not to narrate the whole thing to your child. Instead you may try something like this "The area in red on the map shows us what part of the world the Panda Bear lives in. Can you look at the map and see where you think this might be?" Be sure to make your conversation appropriate for their age and level.
You will also want to consider what kind of preparation you will want to do with your child. In the above example, you would want to start your study of Africa. Where is it? What kind of climate do they have? What animals may we find there? Laying this foundation will let your child relate to what they see and provide a much better learning experience!