Lunch at the Hills...whether in LA or the French Riviera...nothing more trendy and sharp than a suit, preferably in clear and soft colors such as white, beige, or khaki. To make it more casual you could only wear the jacket with a tie and some jeans; or the jacket with an unbuttoned shirt and jeans. Don't forget the girls and cocktails.
This summer the boho/ethnic trend is still going through a revival. Combine boho and mod with some Hindu style clothes; which is never out of style like The Beatles. Nehru shirts and jackets are a must have. The color and decor is up to you but I recommend light and fresh materials such as cotton and linen.
David Bailey knew how to dress on a cool summer day in Cannes back in the day. A navy blue jacket/blazer, a light pullover (white), dark trousers or jeans, and sunglasses (Wayfarers or Persol Rattis). And don't forget the attitude!