M.K.
I hear that the splash pads are fun. There is on in Lombard on St. Charles and then there is Ty Warner Park in Westmont. They have a park, sand play area, covered pavilion, and the splash pad. Google ty warner park and you will find it.
I like the four season park on Main and 16th street. The little kids area is nice and it is shaded, except for the swings.
The Arboretum is always fun, the kids area is really nice, it has some water areas so take extra clothing.
Cosley zoo is great and free.
I know that Borders in Oak Brook and Wheaton have a reading time and day. They read a book to the kids and then do a craft. Not sure which day, but I'm sure you can call them. Great for a rainy day.
Have fun this summer!