Sunflower House: fun to play in once the sunflowers grow tall! I have seen this done with on large square with side "rooms" and an entrance area. Fun for kids especially if they plant seeds and tend .making their own playhouses.
Originally posted as a gift idea for a child. But I think it would be a great way to see how root plants grow. You plant seeds that grow underground plants and watch the roots and vegetables form. Good teacher or parent craft for kids.