Grab a 1 inch, 3-ring binder. If you don't have one laying around the house, you could always get one for $1 at Office Max through the 14th of July. I've also seen them at Wal-Mart for $1 too... and I don't think it was a promotional thing. Also, pick up a package of clear plastic sleeves (if you don't already have some).
Now, check the back of your cereal boxes. Are there great little games on the backs? My brother and I used to LOVE to do the games on the back of the cereal boxes. Start cutting them out when you're done with the cereal and slip them into the clear plastic sleeves. You could also go through magazines for kids and take out the activity sheets. If you don't get magazines at your house, then ask the local dentist or doctor's office if you could have them when they plan to throw them out. It doesn't matter if it's a current issue or not!
Throw all the activity sheets into the binder and voila! You have your self an activity book that didn't cost too much. The best part is that you can change out the activities when your kids get bored with them. Also, if the activity requires writing, a dry erase marker should be able to write on the clear plastic sleeve and then be erased!! Everyone can do the activity!!