Learning a new skill or pursuing a hobby. Bonus if it does both at the same time. If say you like coding on your free time and your job happens to be related to that skill, then you get the benefit of doing both. If say your hobby is a sport and that’s not related to your job at all, that’s fine. The important thing is you’re doing something you enjoy on your free time, and time spent enjoying something is not wasted time. Except drugs, or anything illegal. lol.