Travel Packs are designed for just that, and its fair to say quite probably the most essential piece of kit that you have to get right has got to be your pack.
Travel packs differentiate from traditional top loading hiking packs as they generally have a full opening zip which allows you to operate them as a suitcase as well, one of the most popular and probably important features is the detachable daysack an essential item for any city hopping backpacker!