A few possibles:
Metal stuff
- Trying to keep this legal, but could you put a wallet card type knife in your billfold and slip the billfold in the coin tray (I don’t travel by plane so no idea if this would even work or what might happen to you if the item is discovered so I certainly am not recommending that anyone try it)
- Victorinox mini-pencil (the one from the SOS kit)
- Carabiners and split rings (of course these could be incorporated into your keychain)
- LED light
Plastic stuff
- In wallet, a gift card wrapped with duct tape AND electrical tape (there is room for both, in fact I think I got the idea for this from another ETS member), Fresnel Lens, and Signal mirror
- Tubing
Wood and Paper
- Cards in wallet with useful info, like knots
- Small block of fatwood
First Aid Stuff
- CPR Shield
- Wound Closure Strips
- Superglue
Other Stuff
- Wear cargo pants, and add a heatsheet blanket and/or bivy to one of the cargo pockets (someone beat me to suggesting a pocket poncho as an alternative)
- Eyeglass repair kit (obvious for me as I wear glasses)
- Water purification tablets (to go with the water bag)
- A length of cordage

Would matches in a watertight PLASTIC case make it through? How about a lighter? I haven’t traveled by plane in over 25 years so I don’t know what the security measures are. Do you empty your pockets completely? If so what gets through and what doesn’t? It seems that from what everyone is saying it is very subjective and somewhat based on the area you travel in.