Natural springs... Rock Springs at Kelly Park is magical; it's a natural lazy river you can rent inner tubes and float down.. it's beautiful and a really cheap day of fun. The water is crystal clear but it's cold! But wonderful on a hot summer day. There's also Wekiva Springs and Blue Springs in the area. Blue Springs is famous for huge numbers of manatees during the winter months.
Also Gatorland is fun if your kids like animals (especially reptiles!). But they have a small petting zoo too. LOTS of gators!
As someone else mentioned, SeaWorld is great. If you really want to splurge, Discovery Cove is incredible. I went as a kid and I still remember it clearly to this day. I think it's one of the best experiences in Orlando but it's quite expensive. Food, snacks, and drinks are included with the ticket though, and they cap ticket sales so it won't ever be overcrowded.