When we went to the park on Sunday, we were given a paper when we entered about "Passholder Pizza Party Sundays". It's from 3-7 pm every Sunday through 9/5 in the Old Country Picnic Grove. The price is $11.99 for adults and $8.99 for children ages 3-12 for unlimited pizza (5 varieties), ziti, broccoli & pasta, soft drinks, and desert.
Sounds like a great deal! We don't usually stick around that late when we visit, but if we did, we would definitely give it a try.