Kids love to be busy. It's how they learn. You can provide that learning experience inexpensively and simply. Here's how:
Encourage your children to explore the great outdoors. Research shows that children who are connected to and curious about mother nature grow up with more confidence and quick thinking ability. Go for a walk and marvel at how the grass peeps through the side walk cracks, or count the number of bird feeders along your way. While your at it, count the number of different birds you see, the colours of them, their size etc.
If you're more adventuresome, fill the tub or a basin full of suds and let the children (while closely supervised of course) play to their hearts content finding shapes in the bubbles, watching how suds turn to water, trying to fill a container with suds, you can even make a simple bubble mix of dish detergent and water to blow bubbles with. When the suds are gone, the children will spend hours playing with different apparatus in the water.
Dancing is another freeby that provides a workout at the same time. Play some children's music and let the children sing, dance their silliest dance, be goofy and jive to the rhythm. There is so much to be learned from listening to music, remembering words and singing them at the appropriate time. Heck you can even start a Congo line that goes out the front door, around the yard, and back in again.
Let your imagination provide you and your children with inexpensive fun. Look back to your childhood years and rekindle the magic and the awe of being a kid again.