The zoo has so much more to offer than just a nice place to walk around and look at animals. They offer great classes, camps, and programs for kids as young as 18 months old to adults. Special events include their Walk in the Wild 1-mile non-competitive event, Rock the Zoo concert, and of course Zoo LIghts during the holiday season. You can even host special gatherings, meetings, or parties there. The Fisher Price Little People play area is a great space for smaller kids.
Yes, it is hot during the summer, but visiting the zoo is not out of the question during that season. The zoo is open earlier, and the splash pad is a great place to cool down.
The membership prices are great; I think the basic membership pays for itself in about 2.5 visits. We go that many times in a month, so our membership has paid for itself several times over.