If your bike ever breaks down and needs minor repairs on the side of the road, would you have what you needed to fix it? Building a small toolkit to hold a few odds and ends can greatly improve your chances, and you don't need much more than an old water bottle to construct one.
Bike Hacks shows you how to build a self-contained toolkit out of two plastic water bottles. Essentially, you cut the top off of one to hold your tools, then cut the bottom off another to act as a secure, sliding lid. Then just mount an additional water bottle holder on your bike, and keep the toolkit there for a rainy day. This would also be great for holding your keys, additional batteries for your lights, and your favourite multitool so you're never caught unprepared. For more details, check out the source link.
DIY Water Bottle Tool Holder [Bike Hacks]