A small/travel size bottle of Gold Bond has been priceless....be it for a long, long haul plane flight, train/bus ride, or just excessive humidity somewhere....
I hope I don't need to elaborate on its essential-ness.
A small tube (chapstick size tube) of nick-it. At the size of a little chapstick tube, it takes up minimal space and does wonders for when you accidentally cut yourself shaving and you're in a hurry to get somewhere.
Instead of packing flip-flops for the beach, pool, or hostel shower, I'll wear a pair of Crocs to the airport. Since they're all rubber, I don't have to mess with taking them off at security. I can pop 'em off on the plane and throw on the plane socks if needed. They work great for beach, pool, shower, etc. and now they even have these sort-of dressy kind that you can wear out to dinner, etc. Items like that which serve several purposes rule.