I don't know about other people, 'things' seem to change when camping, so ever only used the bottles. You can usually get decent'ish toilets& showers, although I second flip flops or crocs, also spare tissues just in case the tissue has ran out.
Citronella is fantastic against creepy crawlies and things that bite. The candles are good, dabbing a bit on your wrists and neck seem to do the trick.
We last went away not bank hols but the weekend before. One thing I made sure I bought was a better pillow. Also basics like you'd have at home medicines for children & medicines for adults. Travel sickness tablets are good & you can get them OTC.
When we go out for food we collect salt, pepper, vinegar, red & brown sauce, mayonnaise, salad cream plus mustard. We used to do similar at Costa with sugars. We save the fast food tubs & store everything in them for quick reach. Keep them inside at home though, as they can get soggy in a garage.
Quick snacks are good too, especially for children. If you try and get all the stuff you need, as at on site shops, they charge a premium thinking they need x/y/z and often you miles away from anywhere.
We look at sites to go to, I'm a bit of a snob and try and push for 4* sites, although the only real difference is there might be a club house. The toilets & showers maintained their camping yukkiness.
The kettle is the type that you boil and whistles when done.
Taking a couple of big bottles of water is also handy.
There's so much, I should add we do the camper van thing, not tents.
Travelling around you get to see loads of vans, one really good one I saw houses 5 people.