This is the opposite question to "what did you pack that you didn't use"! I now take a light Target sweater in addition to my LL Bean fitness fleece (which I take for unexpectedly cold weather-by that I mean in the 60's) The sweater can be easily stashed in my purse and looks better in a restaurant, as well as being lighter. It is great in air conditioning.
Also plenty of Tylenol PM. I had a muscle issue in London that really impacted my ability to sleep and thus energy for sightseeing. It was at the end of my trip so I only had one left and nothing I could find in their drugstores worked.
Also a fold-up bag that I can use as one of my two carryon items on the trip back when I usually check my regular carryon.