If you're visiting Miami with children and have rented a car, I recommend a visit to the Miami Metrozoo. Rated in the top 10 in the country, it is huge, has a wide variety of animals and many options to visit it. You can walk, but get ready to walk a lot, so unless it's winter and fresh I would not recommend it. You can take their train and see the animals from above, hearing explanations along the way. Or, you can rent a bike for two, four or six, which I personally find best. The key is to get there, as it is quite far down south. Take the turnpike, it will save you a good 30 minutes!
Oh and the gates close at 4pm, although the zoo closes at 6pm, so be sure to arrive before 4pm!