Years ago, I was told that your carry-on bag should contain just about everything you need to survive at your destination should your main bag get lost (or you end up stuck at the airport for a while).

Therefore, besides cash, a cell phone, and identification on my person, I make sure my carry-on has:

1. Some extra cash.
2. Spare change of appropriate clothing (I usually pack jeans, a long sleeve shirt, underwear, socks, and a hat)
3. A water-resistant jacket
4. Basic hygiene supplies (comb/brush, tooth brush, wet wipes, ect)
5. Water and snacks (used to be able to bring through security, now must be purchased post the security checkpoint)
6. A battery powered Am/Fm radio with earbuds (to get up to the minute news regardless of whether cell phones or wifi is working).
7. Something to keep you busy for a day or two (i.e. a good book, ect)
8. A pen and paper
9 Basic first aid stuff (band-aids, gauze, OTC medication, as well as any necessary prescription medications)
10. A flashlight
11. A Whistle

I used to be able to carry a small Swiss Army knife in there as well, but now I have to put that in my checked bag.