A power bank to charge your phone. Nothing worse than being out at night with a dead phone and not being able to google how to get back to your hotel. Also some sort of mobile internet, either a sim or a portable wifi (I prefer a sim, but with a group the wifi option can make sense).
Power adapters, if you need them for the place you're traveling. Possibly also a power strip if you're traveling in a group. Hotel rooms can be stingy with the number of power sockets.
Earplugs. A lot of hotel walls feel like they're made out of tissue paper, so this are basically a necessity for getting a good night's sleep. And they also help you sleep on an airplane/night train/bus.
(At least) two separate wallets or similar stored in separate places with a credit/debit card in each. Losing a wallet is bad enough normally, but you don't want to be stuck in a foreign country with no access to your money.
A Kindle is I think the perfect entertainment device for travel. Light to carry with super long battery life. Perfect for when you're at an airport/train station and need to kill some time.
If you're on a long flight, an airplane headphone adapter, so that you can use your own headphones with the plane's entertainment system. This isn't that important anymore because a lot of airplanes don't need them anymore, but I still carry one just in case.
Depending on where you're traveling, some change in local currency. Some places in Europe are infamous for paid toilets that only take exact change, and there's nothing worse than being out of change when you need one. One a related note - tissues. You never know when you might need them.
Comfortable shoes. You're probably going to be walking a lot. Also a comfortable backpack given how much you'll be moving between places.
