Natural toys

Dear Friends,
       I am a first grade teacher and always at the end of the year I want to spend most of my time with my students out of doors. This is my science time when I like to play with the elements. I am looking for simple toys to make with children that explore the qualities of the elements. What I know I can do are kites and pinwheels and parachutes to play with the wind and the air. We can make boats of all sorts to sail down the nearby stream but I am wondering if anyone has tips or tricks to make simple water wheels or other thoughts about water toys.
I would also like to make marble runs out of earth and mud or maybe sticks nailed to a board to guide the marbles or other simple gravity exploring toys. Any ideas would be appreciated. Also if any one has any ideas for contained fire toys, or maybe we just roast some snack potatoes.
Anyways, what have you played with as a child out of doors? I would love to hear your thoughts.Thank you!
Warmly,
Joan

Ohhh this just warms my heart. How sweet! And how lucky those kids are for a teacher like you What about grass/ acorn cap whistles?

Hey Almond thanks for your reply. I know how to make grass whistles but how do you make acorn cap whistles? That sounds like something I would love to try.

Acorn Cap Whistles
Maybe they could make an outdoor pretend kitchen or store? Every kid loves pretend and many don't get the opportunity anymore. This book has a lot of cute ideas for that:

A Pizza place where I lived one year had "Duck Races" which the kids loved.

Since you mentioned a creek, is it possible to have "Duck Races" with rubber duck bath toys?  The idea is to have the kids bring a duck or a boat in this case.  These are numbered and assigned to their owner.

Ducks were put in the creek at one location and the first "ducky" to the designated end was the winner and received a prize. This prize could be some sort of trinket.

Maybe a scavenger hunt with different signs to help find the scavenger items?

Rock painting with milk paint?  I like ladybugs.