The item that has occupied my children the most this summer cost me a total of $3.18. It is the squirt bottle. I bought three at the dollar store, and oh how much I have loved them.
All three of my children (6, 4, 2) are enthralled by the squirt bottle. They love to squirt plants, cement, flowers, trees, rocks...they love to see what happens when they squirt and see the designs they can make. They can also squirt themselves and each other when they are feeling too hot.
Such a simple thing, but it teaches some science and works those fine motor muscles in the hands.