We completely used the breakfast food in the hotel trick. We brought a cooler as one of our carry on items and was able to keep stuff cold without needing a fridge. I've heard great suggestions of collapsible coolers, too.
We also bring gum, candy, chips/snacks, granola bars, fruit. I also bring/buy Mt. Dew once I get down there, since I can't get it in the parks.
A couple of pens/sharpies.
Tylenol/advil, cold/sinus meds, blister material/mole skin. All things I've had to purchase while there and were more expensive.
I like to buy pins before going. It really doesn't matter which ones you have to start with, since they are for trading.
I bring lots of ziplock bags of different sizes. I agree about the ponchos, and I'd add an umbrella or two.