I always take high-quality socks when I travel. They can get pretty expensive $15-$25/pair but they are totally worth the price. Plus, mine have lasted for a few years and they're still holding up great.
A mini keychain flashlight always finds its way into my bag.
I also throw in a few plastic zip bag for storing whatever (I put my passport in one just in case I have a spill).
I packed a set of plastic utensils which are great for those quick picnics in the park.
Do you have any "must-haves' that you always pack when you travel?