If you're interested in getting to know how to craft your own electronic devices using Arduino, here's a great project from Instructables that will exercise your brain as well as your legs. It's an Arduino bike speedometer that will calculate how fast you're moving on your bike.
The instructions are incredibly clear, and the parts list even provides links to the exact part you want to pick up for the project online. However, it's a little bit of a pricy project.
This project uses a magnetic switch (also called a reed switch) to measure the speed of one of the bike's wheels. The Arduino calculates the mph, and send this information out to the LCD screen on the handlebars as you ride. It is compatible with any kind of bike/wheel, simply enter the radius of the wheel in the firmware to calibrate the device for your setup.
The first thing I thought of, and that you're probably thinking now, is that you can just go down to the store and pick up a far cheaper speedometer and have the information you want without the work and for less money. But as this project creator points out in the comments, "The joke around the office is that you can't put a price on diy."
It's true you can find this already made for you at a store, but then you'd miss out on the fun of building something yourself, of figuring out how to make it work correctly, and on learning skills you can apply to other DIY projects. A bike speedometer may cost about $10, but what you get from DIYing it is priceless.
And of course you can customize. As one Instructables commenter points out, "You could also pretty simply add a GPS and logging capabilities. That way you could see how fast you're going and where. With that data you could start predicting how long it'd take you based off what time you're leaving and we're you're going. While you could buy a speedometer for cheaper this gives you a platform to add much cooler things."