for the last 3 years , I have backpacked Europe at least 2 times a year for 5-7 weeks at a time
backup your passport
photocopy your passport and just carry that around
only carry 1 debit or visa card around , leave backup card in room
universal adaptor
Belkin power bar
I use pac safe day bag and backpack . they are slash proof
pack lightly , you don't need that much clothes , you can use airbnb washer and dryers
figure out what bank doesn't charge foreign atm charges and open up account there
portable phone charger
download apps : maps.me , google translate . airbnb . booking , hostel , railplanner , whatever airline you are flying on ,xecurrency ,WhatsApp or facetime , viator , tripadvisor ,couchsurfing (great way to meet other travelers and free walking tours or events)
unlock phone