I am a fairly seasoned camper and this would be my list:
Nice sleeping stuff - a self inflating mattress is better than an air bed which just always deflates and if you are on it a double one with someone else you are just constantly moving. Bring your own pillows from home. We use a double sleeping bag for me and DH which keeps us nice and warm.
Good comfy chairs - I have an enormous moon chair which takes up loads of space but I can properly curl up in it and I love it. Small chairs for the kids otherwise they want to sit in yours.
A folding picnic table for eating at.
A folding kitchen thing with a 'cupboard' built in.
Gas stove
Portable toaster (its just two bits of metal you put the bread between and hold it over the stove)
Get a camping set of pots/pans, cutlery, plates, cups etc, so that you don't have to bring your own crockery from home - Decathlon do a set.
Kettle
Tent Carpet to stop the floor of your tent getting manky and also makes the tent more cosy.
Loads and loads of pegs, especially if you are going to a beach place. Maybe some kind of washing line as well, otherwise you are just trying to hang towels etc over guy ropes.
If you are not going electric (we never do) get a couple of those portable charger things for phones, tablets etc
Hanging lights - again you can get them in Decathlon and tents usually have loops to hang them off. If you want to be a camping wanker like me you can also get fairy lights/festoon lights for outside the tent.
I'm sure there is lots more stuff I could think of!