Make a roguelike game. A basic one can be implemented in a couple of KLOC and will make good use of STL, algorithms, and memory management. Best of all when you are finished you will have a fun game to play!
The annual 7DRL challenge in which you have 7 days to make a roguelike is starting next week. See https://itch.io/jam/7drl-challenge-2019 for details.