This is a very easy accompaniment to almost any grilled fish or meat. It's fine to chop all the veggies, toss with a bit of oil and then let them sit in the fridge until it's time to grill.
Any combination of summer vegetable is great - some ideas...
I always pull my meat off and do the veggies while the meat rests. If I'm serving this with roast chicken, I grill the veggies in the pan that I cooked the chickens in!