A credit card.
Seriously, though:
Take more bowls, plates, spoons and forks than you have people. We tried cutting down one year and it was a complete pain, as you need tools to cook with.
What do you plan on cooking? If it's a trial, then don't bother buying camping gear (apart from a little stove), just take pans from home.
Plastic dishes and cups are betterIMO than aluminium, enamel or disposable. But, again, if this is just a trial, don't spend on something you are unlikely to use again if you don't like it. We use our plastic dishes for eating in the garden, parties, etc, as well as camping.
A washing-up bowl is a must, to carry your dirties to the washing area.
We find this style of stove convenient. One canister of gas should do for a weekend.
Dh can rustle up a decent meal for 5 on a single-ring gas burner and a Trangia. I don't know how he does it!
I would buy a 5l bottle of water from the supermarket. If this becomes your thing, then invest in a folding water carrier.
Don't worry about balanced meals with fresh veg etc for just one weekend. Take stuff that won't need a cool box. Unless, that is, you already have a coolbox, in which case take extra ice-packs in a labelled carrier bag. Most campsites will have a freezer for you to refreeze packs.
We have a Tupperware with cutlery, sharp knife, small wooden spoon, peeler, small chopping board, bag-clips, tin-opener, bottle-opener and skewers. I've not found a fish-slice that will fit in it, but then we don't tend to need one.