You're going to take so much pictures and videos that it's so much better to use a camera. You don't want to be scared of your phone safety and want to keep as much battery as you can to stay safe, so having a camera or a gopro is a must have during a trip. A GOPRO will allow you to take some adventurous videos and pictures without never been scared to break it.
2. Little shower towel
It's something people often forget to bring, but is not always furnished in every hostel you can go to. Nowadays, you can find microfiber one, that will be easy to dry and not taking any space in your backpack.
3. Travel pillow
First because you cannot imagine yet the days and the nights you will spend on shuttle during your trip. It will help you sleep better, be more comfy and avoid this situation where you are alsleep on your neighbour (event if it can be an ice breaker).
Having music can transform a good moment in incredible memories. Today, you can buy little speaker that you will be able to pack easily and with amazing sound.
Because every country don't have the same plug, don't forget to bring an adaptator to be able to charge all your devices.
6. Dry shampoo
Because you don't know on what adventure you will go, it's a girl product saver to feel not clean but at least not gross all days long :)
7. Ear plugs
Hostels are great, because you can party a lot and meet a lot of new people but when you're tired and want to sleep, it's cool to be able to put yourself in a bubble.
8. Travel padlock
In hostel, you don't want to be preoccupied about your stuff and you often have access to cases were you can put your stuff so don't forget to have your padlock to lock them down.
9. International sim card
Not a lot of people know about this sim card for 30€ a month you'll be able to have access to social media like messenger and whatsapp unlimited whithout worrying about changing card in every country you visit. A save of time and money and energy :)
10. Sleeping bag liner
In hostel it can be a bit dirty, or you have to pay extra for the sheets so it's cool to be prepared. Also it can be cool in night buses where they put the AC very hight or for any adventure you'll be in.
Always be prepared to be able to say yes to everything. Don't forget to enjoy your trip and that it's part of the plan to not having everything, you'll be surprised by the fact that you can manage everything with almost nothing.