The time-honoured carabiner trick for quickly sticking bags onto long-drop locks.
Some chunky glowsticks for putting on top of any piles of bags on the floor once it gets twilighty and people are (fairly enough) likely to trip over them if they can't see them:
(bring enough and bring string for everyone in your group to wear around their necks later on, fun in the SEC, and it makes it way easier to find people in crowds at night)
Medium sized blue IKEA bags for organising your tent's storage, packing cubes of clothes in one, toiletries in another, party bits in another etc.
These eBay/AliExpress pumps that attach to a standard fizzy drink/mixer bottle for showering (you can adjust the nozzle to be a mist or a jet):
These AliExpress hooks are perfect for clicking onto the fibreglass loops/poles of Quechua pop-up tents to attach things like solar powered lights, bunting etc: