The top section of one of those plant squirter things - a lifeline if it's hot, and you don't need the whole thing as it takes up too much room to pak and if you need it you can just screw it on to an evian bottel as it's a universal fit
A pillow case - judging by your sleep gear you've already got you're a big sleep fan like myself, well a pillow case turns any camping pillow, or an bundle of clothes into a normal-ish pillow, and the temparature of the pillow case (v important) will be similar to that of your home ones.
Loo roll you can get there
J cloths (something always needs a wipe)
A folding bit of tarpauline/showercurtain/anything-that-can-be-folded-up-small that can be used as a mat/picnic blanket
Drinking utensil (cup and/or mug - the ones you get on site are all flimsy and my mornign cuppa deserves a proper mug Ditto pints (cider excluded - that MUST go in a paper cup)
I could (genuinely) go on!