For the last month or so, I've been wanting to take Cal to the New York Hall of Science, which is this really cool science museum in Queens. I went a couple of times when I was a kid, and being a young nerd, I really enjoyed myself. I mean, come on now. SCIENCE. What's more fun than that?
Anyway, one thing I especially appreciate about this museum is that though it is clearly geared towards kids (almost every single exhibit is hands-on, if not climb-on), the concepts and explanations are surprisingly high-level. The exhibits about optics and mathematics were particularly interesting for adults, I thought, and I think there's no kid alive that can resist the allure of gigantic soap bubbles or a hall of mirrors. If you live or are visiting anywhere near Queens, this is little side trip is worth it. (Full photo set here.)
Also, on that note, thanks all for your suggestion of kid-friends activities in or near Atlanta. Very awesome recommendations, we'll definitely be trying many if not all of them at some point.