Youtuber Spiritplumber devised a cheap and relatively-simple way to extend your cell phone's battery life. Recording video on smart phones is notorious for draining your juice and he claims this workaround can last a few hours with just a couple of bucks. Or you can go the less ugly but more expensive route and just pick up one of these guys.
A few notes: This is meant to be a visual guide, as many similar outlets have different voltages. Just because a plug fits doesn't mean it will work. Always double check before you blow up your cell phone/hair dryer/discount lightsaber.
The outlet widely-used in continental Europe is known as the Schuko and varies between countries; G