I'm planning to take a backpack during my day trip with kids to NYC. No museums on the list for this trip, but am planning to go into some stores, public library, and possibly Central Park Zoo if the weather is decent enough. Should I be aware of any issues foreseen with carrying a backpack? I'm not keeping money or valuables in it, just possible snacks, bottled water, and items we may purchase throughout the day. I appreciate any input.