Forgetting bitcoin for a second, if you just want to polish up your C++ skills, a great free site to do this is hackerrank.com , extra cool because you can do all the various exercises in several different languages back and forth.
amazing site, another one like this is hcodecademy.com, but I think that hackerrank.com has more practical and interesting exercises. It is completely different if you just read code of bitcoin core or you actually code.