Other than my camera there’s just one thing that I bring with me nearly everywhere I go, my Klean Kanteen.
Like Vibram FiveFingers, I’m a big fan of Klean Kanteen and their BPA free (healthy), reusable water bottles. Buying and drinking bottled water is extremely wasteful, typically less clean than tap water, and a waste of money. (For an amazing, eye opening documentary on bottled water watch Tapped)
The issue I’ve run into in Europe though is that there’s no such thing as ordering tap water in restaurants.