Many travel lessons are learned the hard way. You should always have a collapsible backpack and a kind of day bag, like a tote. When you work hard to travel light, you will find that you can't just empty out your primary carry-on and use that for touring. You need to have a smaller bag to throw your daytrip things in and go. I have this REI collapsible backpack (used in Peru during hiking the Inca Trail and climbing those god-awful steps to the Machu Picchu entrance) and to use when both hands are needed during hiking / walking and you need to be more evenly balanced. For city daytrips or touring, I use a small canvas bag. It's all about preference --- but an inexpensive collapsible backpack will almost always serve you in good stead.