First it has to be interesting to me, Second is it going to bring me money (job, side project) or just joy (hobbies): Few examples:

1) my job systems/security and my side project: stock trading robots

2) fishing, sailing, Cuban cigars, old scotch, wine, reading, sports, etc...

And one in between: Art collecting....

You are living only one life and not very long at that (so far), so don't spend it on boring stuff...