The Apple Watch can be used to control cars, smart devices, and everything in between. You could always develop custom solutions for it. planetleak has posted an article on Instructables.com that shows you how to build a media controller app for the Apple Watch. Sure, there are better media controller apps available, but you will learn a lot by trying this:
You are going to need an Arduino, Xcode, and a few other items to get started. More information is available here.