The Swiss Army Knife is a good idea, but one should probably pack this in the checked luggage instead of carrying it on a plane. When you go through the X-ray Security gate you'll have to show this knife and, depending on security, you may or may not be able to take it on the plane with you.
A few nutty things I take and rarely use are the larger size paper clips to hook together two "meeting" zippers on a backpack/bookbag. Makes opening them by "fast hands" extremely difficult and gives you more peace of mind.
Also, don't forget a couple of grocery-sized plastic bags for either your shoes, dirty clothes, for putting wet/dirty/potentially-messy items. The wet-wipes are always VERY handy on the plane, on hot days, or for hands if you can't get to a restroom to wash before buying a sandwich on the street.
ALWAYS take/carry one small-sized notepad and a couple pens for important notations taken about train/bus times, local bus numbers, to write directions or phone numbers of cute girls you meet along the way. hehehehehee...
A couple larger zip-lock bags are good too. Always, towards the end of a trip I accumulate A LOT of little papers, receipts, flyers/brochures/maps, business cards, etc. and this is good to keep them all together. Take rubber bands too!
I always suggest wearing shirts (at least for men. Women don't need to wear them. heheehe) that have one or two front pockets for airplane tickets just before boarding. MUCH easier than fumbling in a bag to find it and much easier to store the "stub" after walking down the ramp to the plane.
That's all I can think of. Anyone else?? By the way, do those blow-up neck-cushions really help you sleep more comfortably on airplanes??? My sister swears by them, but have never tried them myself. I always giggle when I see someone stand up to get something from the overhead compartment and they still have that silly looking thing around their neck.
Saludos, MadridMan