I have arrived on many occasions to find my suitcase has not travelled with me (worst case a 7 day river cruise when the baggage arrived on day 7). My carry on baggage therefore includes 3 days of underwear, shorts, long trousers, 3 tops, 3 dresses, cardigan, nightie, sandals, socks, shoes, bikini, spare spectacles, sunglasses, scarf (to cover hair in Muslim countries and as a sarong poolside).
For immediate use handkerchiefs, toothpaste and toothbrush, moisturiser, lip balm, medication, comb, iPad, phone, books, pen, snacks or even a packed lunch if it's a cheapo airline. These go in a bag which I remove from my carry on luggage and place under my seat to use during the flight. I hate people rummaging in the overhead locker mid flight, so dangerous.