The second (round) hairbrush.
I am a fanatically light packer. I'll wear the same three blouses and two skirts for an infinite number of days on business trips, have no problem using airline amenity kit toothpaste and moisturizer as well as hotel room shampoo and conditioner, and don't wear makeup. But I never travel without that round brush.
I also bring my running clothes and sneakers everywhere. The sneakers take up a lot of room in a 20-inch rollaboard, but I manage to fit everything anyway. It helps to be small and have small clothes!