Going with c# again. I know the language super well but don't often have a chance to get really deep into it with the stuff at work. These often present very non typical problems that require lesser used features.
I'd advise to pick up at least a second language for your non-critical projects. At first it feels so uncomfortable, but as you gain proficiency, you'll see it's a real ego boost. You realise that there really isn't anything you can't learn.