It may seem unconventional, but when you are an avid traveler you'll find the best way to never forget your vitals is to never move them. From travel-sized toiletries to a set of chargers, packing will be a breeze if you are already halfway there.
What else is pertinent for a fully primed carry-on? Start by thinking about past travel experiences and the items you have cursed yourself for not having. This may change based on the person, but an umbrella and other weather gear is usually a safe bet for all. Even if you are headed for Bermuda, having an extra pair of warm socks and a rain jacket could save you at some point. Airplanes are cold, weather changes, and because it's always better to have it and not need it, than need it and not have it.
This summer when you are packing light for a Spirit Airlines flight, do yourself a favor and keep your bag pre-packed.