I'd echo the door mat (or at least a sheet of cardboard)
I have plastic pegs that are purely for camping purposes - being plastic, I can wash them if necessary, and I use them for hanging towels etc and also for closing food packets if I have run out of clippits.
Another Leader in our Scout Unit swears by the large Blue ikea bags as "suitcases" for his DCs on camp - they squash up much better in the car, and allow easier rummaging by each person in their own bag (more space and light to see into than a zipped sports bag), and anything thrown around the tent can be put back on top of the relevant bag by adults clearing up after a rummaging event.
Has anyone mentioned the filled 5l water bottle and head torch turned inwards trick to make a lantern yet? (Sorry, only on p.1)
I also bring spare Ziploc bags to use for marinading meat, storing food, making pancake mix (cut off tip of bag to pour, clip that with clippit or peg if leftovers to use at a later meal), etc.
Woolly hat for sleeping, and a pair of warm woolly socks that are strictly only for sleeping bag use.
Insulate underneath your bed from ground - those foam tiles from playrooms, a spare blanket, tent carpet if you have it, foil backed picnic rug etc (whatever you have, not all those at once!). Put a layer of something underneath your sleeping mat/mattress and sleeping bag/duvet set up.
Carbiners and S hooks to hang things in tent, around tent, and to hang washbag and towel etc in showers.
If you have a plastic bag type "camping shower", put it in a sunny spot in the morning, to get whatever heat you can into it, and you will have hot water for hosing off or doing washing up later. Takes a good while to heat, but handy for "free" hot water - the roof of a car in full sun can be a good spot if you are not using the car for driving.
Small dustpan and brush are invaluable for keeping tent clean (ish).
And never ever go camping without a roll of tin foil for cooking all sorts, making lids for pots, wrapping lefotvers etc.