It is always smart to pack a spare set of clothes in your carry-on luggage in case the luggage you check at the gate is lost. That way if you get stranded at a destination without your luggage, you have a spare set of clothes to change into when the clothes you are wearing are no longer fit to be worn. Pack for the climate you'll be traveling to, as well--after all, being stuck in New York in mid-December while wearing only shorts and a T-Shirt is far from comfortable.
Because of security measures, you may not want to go through the trouble of bringing liquid soap, shampoo or deodorant spray with you. However, you can still bring bar soap and stick deodorant with you in your carry-on luggage and, should you need to freshen up or be stuck at the airport overnight due to a delayed flight, you can quickly clean yourself up a bit.
If you are traveling internationally, it is a wise idea to take some prepaid phone cards with you in your carry-on bag. These cards can be found at most airport gift shops and can be a lifesaver if you find yourself on an overseas trip with your luggage nowhere in sight. If you have one and you haven't thought about keeping it with you because you're at the airport, consider putting your cell phone in your carry-on bag as well.
Contrary to what some are led to believe, you actually can bring food with you through security so long as it is in your carry-on bag. Putting a snack-sized bag of potato chips in your carry-on may sound slightly absurd, but when you are on a cross-country flight and your options are to either reach for the snacks in your carry-on or pay $6 for a bag of peanuts, you'll be very happy that you brought that candy bar with you.
