I will be entertaining 5 boys this summer ⎘, 10, 9, 8, &2 } and am looking for free or reasonably prices activities to do with them during the daytime hours. I am from the Alton Il area but would drive to Mo. If anyone has any ideas I would love to hear about it.

The St. Louis Zoo is always a great activity for Free.

Going to a park and packing a picnic lunch is a good one.

fishing or bikeriding. 
Scavenger hunt in the yard/nature walk.  Playing baseball, soccer, basketball.  Catching lightening bugs. 
Flash light tag and bubbles and sidewalk chalk are also entertaining.

I'm not sure if you have a cheap movie theatre in your neck of the woods but we have a $3 show.

Water play is also great, just hook up the hose one day and a slip and slide and you could buy a pinata and make water balloons and do a balloon toss game.