We took a little trip to the Philadelphia Zoo last weekend with some friends — Nick got a year-long membership for his birthday last August, which includes entry for the two of us, along with entry for two guests, AND free parking! It's a great deal, and we have already gotten our money's worth (or, rather, my mom's money's worth, since she's the one who gave the membership to him!).
The Philadelphia Zoo is America's oldest zoo, and if you're ever in the area, you should check it out. Going while the weather is still cool is a plus if you have the option, since it's never quite as crowded as it is in the summertime.