Dorney Park in Allentown, PA is clean and nice. When I was a kid it was a little, cheesy, family-run place but now it is owned by Cedar Fair and is lots of fun. Grandma and Grandpa live near there and we go almost every summer. They have two beautiful carousels.
Hershey Park is my favorite after Disney. It's green and clean, and the "characters" are 6-foot tall candy bars! Plus you can smell the chocolate being made from the factory. YUM!
Kings Island in Cincinnati is also big and colorful and lots of fun. Boy, it is expensive though. With parking and food it may cost more than WDW.
The park in Twinsburg, Ohio was really nice when it was a Sea World. My kids loved it. Now we go to the one in Orlando!
For family-oriented, clean, affordable fun, can't beat "Holiday World" in Santa Claus, Indiana. The same family has run it for years and the parking and DRINKS are free! They have all these little stations where you just go up and get your drinks. Wish Mickey would do this! Last summer we were there and it started pouring and they came around giving away ponchos... Again, couldn't imagine Disney giving away ANYTHING! But as fun as these places are, NONE measure close to WDW!