Spare tent pegs, for when they fall foul of the tent peg eating field or some ignorant pratt who having realised they've forgotten theirs decides to "borrow" a couple from other peoples tents WITHOUT ASKING! A mallet to bash the tent pegs in with and something to lever them back out again when they appear to have become "one with the field". Spare guy rope to replace the ones that get tripped over and snapped.

Bottle for water, if only for when you wake up convinced that some strange furry creature has overnight taken up residence in your mouth!

Things to make a cuppa before anywhere's open (or after everywhere's shut.

Wet wipes or something similar ... too many reasons to list.