Preferably something that can be measured with milestones (e.g.: foreign language exam, either you pass or you don't), "stretches your brain" in a different way (so if you already know Java, picking C# may not be that much of a stretch), and reasonably useful for your life (but not necessarily work).Mine so far:
1. Aiming to pass B2 exam for French (meets all 3 criteria above)
2. TLA+, hard to measure progress, stretches my brain in a different way, but practical utility is dubious
3. Category Theory, similar to TLA+, but even harder to find pragmatic applications.Am keen to hear from others.