[Expez]

In his book Flow, Mihaly Csikszentmihalyi dives a bit deeper into what sort of "problems" we should choose for yourself if we want a happy life. The gist of it is that we should pick activities that we can loose ourselves in. Everyone's experienced this, where they work on something, look at the watch, and a surprising amount of time has passed. A few examples of good "problems": learning an instrument, coding something or learning a sport.

I highly recommend reading the book, as he goes into the key features that get us into the pleasant flow state, so you know what variables to tweak if you can't get there.