Actual project work solving actual clients’ problems in an area that matches your probable future employment. Consider doing something for free (without deadlines) just for experience.

Client problems are typically very different from exercises in compiler or rendering architecture, or whatever, unless you happen to be striving for those exact niches, which just might be a pipe dream unless you’re spectacularly talented in abstract problem solving in low-level languages on multiple hardware platforms and capable of furthering those technical niches in some way that someone benefits from enough to keep employing you.