The best advice we can give you is to travel as light as possible.
These are the main items you need to take:
- Backpack + day bag
- Travel Documents (Interrail pass, pens, seat reservations, passport, travel insurance)
- Money (Cash + Card)
- Clothing (everyday clothing, comfortable shoes, flip-flops for showers)
- Bathroom (toiletries, towel)
- Electronics (chargers, adaptor if needed, power bank)
- Others: padlock, water bottle, blister plasters
You can find here the Interrail packing list which can also help you.