I have a backpack that has a 2 person picnic set in it - plastic plates, wineglasses, cutlery, tiny cutting board and sharp knife, corkscrew. Got from a camping shop.
I rarely actually manage to use it (we are a family of 3), but throw into it a couple of plastic tubs we have for packed lunches with things like a mixed salad, pate and crackers, prawns/squid rings and some dippy stuff, cheese (usually presliced but have the knife to bring a block), cooked chicken legs/wings, cooked sausages/sausage rolls, hummus and vegetable sticks to dip etc. (Not all at once!).
Add in a packet of nice crisps, some fresh fruit (sometimes whole, sometimes already chopped into a tub to pick at), maybe something sweet if I am organised (when hiking, I do tend to butter slices of brack and put together in a Ziploc bag, butter sides facing, which is nice then but might be too heavy for a summer beach trip) or just a packet of biscuits (non chocolate for hot days).
For drinking, I usually fill reusable water bottles with elderflower cordial, nice lemonade (sometimes just bringing the lemon base and a 500ml bottle of fizzy water to add to it when we are ready is good), or sometimes just a couple of cans of San Pellegrino drinks. If very hot, I will put a lot of ice into drinks rather than water at home, as that will then melt.
A few nice paper napkins - the kind with a nice summery pattern on them, and a bag for rubbish.
And a rug to sit on - we got one from Lidl in the weekly specials aisle, but I know I've seen them in camping shops too - ones that roll up into themselves with a handle to carry on the outside, and a foil backing to keep the chill and any damp from the ground away from you.
