Trying to stick with seasonal ideas, I have many more photos of this experience on my other computer, but wanted to at least get the idea out while there are leaves on the ground (for some of us).

This was last year when O 4 months.  I raked a big pile of leaves and just laid him down in the middle of it, both on his belly and his back.  I showed him how the leaves crunched and he started grabbing at the leaves, crunching them and of course tried putting them in his mouth.  I would also pick up a bunch and drop them, he would just lay still watching them as they fell.  

He laid on his back for a surprisingly long time.  I wondered what he was thinking, every time he moved there was crunching.  He most likely didn't realize it at that age that he was controlling the sounds but was certainly curious about them. 

All together he was in the leaves for probably close to 15 minutes.  There were a few position adjustments but for the most part he was just fascinated and content.