So we end up going here every year and we bring my son and my nephew and have a blast! The entry fee is not that expensive, and they usually have some kind of promotion going. The last time if you brought in a Pepsi can, you got $5.00 off.
They have one price, and you can go on all of the rides. Obviously games and food are extra money, along with some other items.
If you go during the week, there is almost no waiting and you can ride all the rides as much as you like.
They have rides for kids (and adults) of all ages and everyone will have a blast! It is very close to our home (about 3 hours) and it can be done as a daytrip.
They recently built a indoor water park nearby, with a hotel attached. We haven't stayed there, but that could be a option for those coming in from far away. It is a little pricey to stay there, but when you have kids, it's worth it.
Update On Apr 01, 2008: Also there is Splashwater Kingdom attached, which is a pretty good sized water park, located towards the back of the area.
Admission is included in the entry price, and is great for the hot days. There is a wave park, tube rides, etc. and have other small water areas for younger kids.
This is almost always packed on the hot days, and I would recommend arriving at the beginning of the day and get in first. This area opens about 2 hours or so after the park is open, so you'll have time to explore the main park first.