Did you do this when you were in school?
First, you go for a walk to gather fallen leaves. Try not to pick the really cool shaped leaves that haven't fallen yet off the trees; they'll give you something new to look at in a few weeks. Pick some leaves that aren't too crunchy. Lay them out out on a sheet of paper. Put another thin sheet of paper on top, and using the side of an unwrapped crayon (block crayons are perfect for this), you rub till the design shows up. Even a toddler can do this and will be very proud of the results. Then you can paste the design to a folded piece of construction paper and send Grandma a "Happy Fall!" card.