Don't put away the winter woollies - you need a hat for sleeping in, warm socks that are only for the sleeping bag, and if you have thermal layers they are extremely useful once evening sets in unless it is the absolute height of a heatwave summer.
Don't get cold in the evening - it's harder to warm up. So once the sun is setting, put on longer trousers and an extra layer on top. (I tend to put my thermal layer on then under my clothes - so I don't need to get down to bare skin when changing for bed - that was a gamechanger realising that!).
Have a pair of crocs, flip flops, shower/slider shoes.....something easy to slide your feet into for going to the toilet block especially during the night. And YYY to keeping muddy boots on a bin liner or similar (I have a rug inside the door of the tent for mine - when DH hasn't stolen it for the house 
- but I usually bring food or other supplies in a carboard box and then use that cardboard for the boots space if the rug is missing).
Under the airbed, put a layer to insulate from the cold ground. Tent "carpets" are good, but expensive. I used a picnic rug from Lidl for a long time (rolls up and ties together, has a silver insulating back which makes it waterproof too - so very useful when camping), and got a pack of kids foam play tiles cheaply a couple of years ago, or even a spare blanket is good.
Have some entertainment for if it rains - cards, board game, books etc. Electronics (with headphones) could be good but need charging so either pre-charge power banks or plan to charge in the car while going on an outing. Find local outings to the campsite too for both good weather and bad - nice hikes (which could be fine if not too heavy rain and you have good gear), or museums, stately homes, adventure parks, even the cinema can be useful sometimes. Also identify potential places to get a takeaway or go in for a meal/coffee if the weather is against you.
"S" hook for hanging on the door of showers - sometimes the actual hooks are broken or there are not many, so being able to hang towel, clothes and washbag is very handy. And use a washbag that is designed to hang as that's much easier to deal with - often floors can be wet (or muddy) and you don't want to put it on the floor.
Even if you are only doing a small amount of your own cooking, and buying fresh food daily, you will probably need some sort of cool storage. The solid cool boxes tend to keep the cold better than fabric ones, but a large one that is fairly empty is difficult to keep cold so only go for the approximate size you need. You could freeze a meal to bring with you to help keep it cold initially as it thaws, or freeze a bottle of water/bag of wine (the kind you get in boxes), and often it is possible to buy bags of ice in supermarkets. Many campsites also have a freezer to freeze proper iceblocks - clearly mark yours with your name before putting in the freezer. (Some people will bring bottles of plain water, 3/4 full (to allow for water expanding as it freezes), and use those to refreeze - but some sites don't allow this).
I always bring a flask, because if I have spare hot water, I might use it later for washing up/hot water bottle for bed/cup of tea etc.