Small bottle of hand sanitizer.
I second, much of what sven said, put postcards into your journal of the places you visit. Take a roll of tape so you can tape random stuff into your journal, ticket stubs, leaves of plants, etc. Turns your travel journal into a journal/scrapbook, very cool.
I never go anywhere without a sarong; bedsheet/ beachblanket/sunprotection/beachcoverup/bathrobe/skirt/shawl, etc.
Take a postcard of where you live to show your new friends, if you live in a cold country, take a snow scene.
A collection of; salt and pepper packets from the take out counter at Wendy’s, catsup, vinegar, toothpicks, hand wipes etc. Takes up very little room but can make a big difference when you need some.
Bottle of eye drops, lip balm, both very small and very handy to have when desired.
Hope you have a great time!