A lot of older European cities and towns have cobbled streets which can wreak havoc on wheeled bags, so one that can be worn on the back with good support structure is ideal. Most larger backpacks have wheels as wheel when you can't be bothered wearing it on your back.
I got the 80litre Deuter backpack. Haven't tried it yet, but it should have ample space. Make sure you take a lock or 2 to lock up the zippers. Valuable items should be kept in a safe place. I wouldn't bring expensive jewellery or anything too precious.
Dirty and wet clothes can be a hassle. On my last day of my last trip I looked like a homeless person with a big black garbage bag I lugged around until I got to Wombats in London and spent some considerable time in the basement, washing. Fun times!