Arduino and other micro controllers. First of all it's really fun (YMMV). There is something about writing code that makes things happen in the physical world which is satisfying in ways that writing code that just affects bits on a computer isn't. Secondly it's one of the realms where C is still genuinely important. When you are working on problems where a few hundred bytes this way or that is difference between success and failure you really start to appreciate what C has to offer.