If you ever ride your bike at night, lights are an absolute must. If you don't have one handy, or just feel like tinkering, you can make your own with a small spice jar and a few spare parts.
You'll need a spice jar with a plastic lid, an LED bulb, watch battery, some eye bolts, and a few other odds and ends. The light will attach to the screws of of the eye bolts, and the battery will wedge in between the eyes to power it. Once you have it working, a pair of hose clamps will keep it secured to your handle bars.
This won't be incredibly bright, so it may be better-suited as a warning light to motorists than a true headlight, but it still looks fun to make. If you aren't a fan of this design, you could try making one out of a garden hose adapter instead, and then create your own turn signal apparatus with an arduino.