I'm heading off travelling around the world in August and need something to carry all my stuff in. I won't be walking much with it, but will be catching trains, aeroplanes and buses, a built in daysack would be good too. I know absolutely nothing about what to buy, but I want to carry somewhere between 15 and 20kg of stuff (with 15kg being the target). As I'm going travelling for a while it does want to be comfortable. Ideally I'd like to spend around $100/£50, but if its worthwhile I'm prepared to spend a bit more.