If your kids enjoy being outside, like playing with water, and are curious to ask questions and investigate answers, this is for you.
Grab a roll of aluminum foil, roll it across the yard, and gently form it into a shallow canal. Your kids will learn after a time or two (or ten!) that punching the foil into the grass results in ripping the foil and starting all over again. Slow and easy builds the foil canal.
When the foil canal is ready, lay a hose nozzle at the top, and turn the water lightly on. Have your kids find lightweight objects they think will float down the canal. It's a scavenger hunt for them, really, because they don't know what will and won't work and need to use their creative juices.