This is one that my mother taught to me and is always popular with children and adults.
Remove a long leaf from a stem of Common Reed Phragmites australis
Fold the leaf a quarter of the way along from one end and then make two tears to divide the fold into three equal tabs
Put one of the outside tabs over the middle one and into the fold of the opposite one (as shown below)
and then repeat at the opposite end
Now the boat is completed
The boat is ready to be floated on water and will blow around in the wind