My son and I went to the Philadelphia Zoo yesterday. There were only a few people there and it felt like we had the whole place to ourselves. People forget about the zoo in the winter, but it is one of my favorite times to go. There are never crowds and the animals are generally still pretty active. Wear your winter gear for sure, but you will be walking around and will generate a little internal heat too. Plus, when you need a little warm up, you can visit the animals in the indoor facilities like the Reptile House.