Great thread and so glad I found it!

I'd like to make my contribution to the list:
microfiber cloths
I'm packing a couple small ones (I got a bag of them in the car wash aisle at Wal-Mart for $5) in ziplock bags in my park bag. I'm going in September and since it usually rains a bit then, these will come in handy in drying off a seat or my feet.