Heading to Oslo and Amsterdam later this year for a week. I'm going to pack a traditional suitcase, but I'd like to find a backpack for a carry-on and daily walking around the city. I'd like it to be small enough to be a carry-on on airlines, comfortable to wear several hours per day. What do you guys recommend? I've read good thing about Tom Bihn? And my daughter has a smaller Osprey bag she likes? And what's the size range in liters I should be considering? Thanks very much in advance!