A first aid kit is definitely a good bet. Also, I lost the bottle with the pin-up girl on it. So there's that.
I'll have to expand on this list because it doesn't include a poncho, a patch of fake grass, a wetsuit bag, a extra wax, and the hella obvious SUNSCREEN.
One day I might have to put this into a proper surf instructional book but only for San Diego. The rest of the cities around here I can't surf for beans there.