The Children's Discovery Museum in uptown Normal has three floors of activities for children and is perfect for ages 2 through 8. I haven't seen many bigger kids there. It's $4 per person over age 2 and opens at 9 am. We were there at 8:55 and were the first ones let in. It was neat. By 11 am the museum really fills up. We enjoy the car and train exhibits (of course) as well as the toddler area, the farm equipment exhibit, and the water tables. There is a really cool climbing structure on the second and third floors that is a magnet for the 5-8 year old crowd.
Tipton Trails sprinkler park is worth the trip. There is a nice shelter with picnic tables and bathrooms and a large sprinkler area with 6-8 different sprinkler zones. Next to it are swings, toddler sized play equipment and a very large barn/fire station themed piece of play equipment. All this is free and draws a crowd on hot days. It's located on College west of Airport Road and is free.