Replace your flashlight’s bulb with three bright white LEDs! From Popular Science: A flashlight with at least three AAA, AA, C or D cell batteries hacked to use three 2300-millicandela LEDs will be as bright as an incandescent and last 5 to 10 times longer. Of course you can add up to 20 LEDs (as long as they fit in the reflector) if you’re planning to, say, man a lighthouse with the thing.