Porting and maintenance. Take an existing code base (your own, open source, etc) and make it run on another platform. Go fix some of the bugs in the code base

Porting and maintenance. Take an existing code base (your own, open source, etc) and make it run on another platform. Go fix some of the bugs in the code base. Then you'll start deeply appreciating what good and bad code looks like. You'll see how helpful comments actually are, how layers of indirection help or not, and all sorts of little details that help or hinder your efforts.