I honestly don't think pleasurable and mentally straining or mentally challenging activities have to be mutually exclusive. In fact I think there's an interplay between the two (sometimes). Think of all the people who enjoy playing chess as a hobby. That's mentally taxing and is still fun to a lot of people. Or crossword puzzles. Or something like that. Those activites are both activities that are not necessarily considered "pleasurable" in themselves, but they provide enjoyment to a lot of people.