There has to be a potential endpoint or else there's no meaning. Pick a hard problem which you find intensely meaningful and take small steps each day towards solving it.

As Karl Popper put it:

'I think there is only one way to do science – or to do philosophy for that matter; to meet a problem, to see its beauty and fall in love with it; to get married to it, and to live with it happily, till death do ye part - unless you should meet another and even more fascinating problem, or unless indeed you should obtain a solution. But even if you do obtain a solution you may then discover to your delight, the existence of a whole family of enchanting though perhaps difficult problem children for whose welfare you may work, with a purpose to the end of your days'