I used an entire roll of heavy duty aluminum foil to create a stream, and used milk caps as boats floating down stream. I put it down the driveway so that it would go down hill.
I then encouraged the kids to play with it and experiment with the way the water moves and how tall they could make the side before the water busted through.
They also put rocks in it for the boats to go around, They put rocks underneath to make small hills for the water to go up and over (that didn't work at all.) We used nature items to see if they would sink or float (grass, leaves, rocks, twigs etc..) We also raced different objects to see what would be faster.
We tried oil to see it separate, we added soap to see if it would bubble and my favorite was adding drops of food coloring at the top and watch is disperse and travel down the stream, that was really NEAT!