While the website suggest a nature walk, a variety of pre-collected leaves could be used during a festival to teach children about tree classification via leaves using a guide book and a dichotomous key.
Find experimental details at Wonders of Our World.
Seminal Science: How Many Seeds Do Different Fruits Produce?
Using many different kinds of fruit (apple, pepper, tomato, cucumber, squash, kiwi, melon), children can investigate the number of seeds per fruit. Discussion can revolve around whether the seed number is proportionate to the size, how new fruits come from seeds, etc. Further investigation can determine if seedless fruits are actually completely seedless or whether each fruit of the same kind have the same numbers of seeds.
See full activity here: Scientific American: Bring Science Home
After a discussion of how and why plants seeds might travel to new locations, children can observe a variety of plant seeds and hypothesize the method of travel for each seed. They may test their hypothesis using a container of water, a hand held fan and a sock.
See full experiment at Wonders of Our World.