This is the simplest boat of all to make. All you need are some big leaves and a long bit of grass or straw.
Collect a few big curling leaves together - sycamores are good - and put them together so they form a curved boat shape.
Tie the leaves at one end with a long bit of grass. Then stick another leaf in as upright as you can to make the mast and sail.
Put some
poppy dollies or miniature grass people on deck and launch your boat downstream.
Then make a few boats and have a leaf boat race to see which one passes a certain point first.