IMO, OP's problem isn't that he/she can't decide which side project to do, it's that they have already decided that the side project involves coding, and they're already a good coder. Doing your job as a side project seems...boring. Pick something you don't know how to do, but wish you did. Learn CAD, use a bandsaw, 3D print a D20, or just take your bike all the way apart. This idea that every software engineer needs to treat software as their hobby is frustrating to me. Bankers, lawyers, and doctors don't do that, why should software engineers?