3D printing, mainly because you can use it to amplify just about any other hobby, and maybe save some money printing things you would have bought. Sure, you can print goofy little doodads and shit, but there are infinite practical uses as well. Get good with 3D modeling, and now you can transform ideas into physical concepts. And let me tell you, there is no feeling or sense of satisfaction quite like holding something functional you designed and made yourself for $2.50 in materials and a few hours of your time. Especially if you saved yourself a few hundred bucks not buying something new.