I do a brilliant greek salad if the weather is nice. It is a Nigella recipe that is fab. you can pre-prepare most of it and throw it together atthe end. It also 'lasts' all afternoon!
I would also do a pasta salad, a rice salad, couscous salad.
Couscous salad, get 2 boxes of Sammi's couscous, any flavour, put in a bowl add correct amount of boiling water and cover. Chop loads of cucumber, cherry tomatoes, peppers etc. Once couscous is done (10 mins or so) add veg and mix. You can do this with couscous and roast veg that can be done the day or so before. Chop loads of butternut squash, red onion, courgette, peppers, sweet potatos and roast until soft and yummy. add to couscous.
Pasta Salad with pasta, red kidney beans, sweetcorn, peppers or whatever. Dressing of natural yoghurt mixed 1/2 and 1/2 with mayo (makes lighter dressing than mayo on it's own)
Green salad - open bags, throw in bowl, add chopped spring onions, cucumber
Tomato salad - slice NICE tomatoes, sprinkle with basil;, balsamic vinegar, black pepper and sea salt (and olive oil) Can add slice mozzarella and black olives.