Saint Exupéry would read and sometimes write while flying, and apparently circle his destination until he’d finished reading his chapter!
I think that was before controlled airspace and possibly before ATC radar 
I do sometimes wonder whether it would be smart to read a book or some such, but let’s say one of the cylinders starts to seize up or whatever. You won’t notice until the situation is quite bad. On one long flight back home I did get the laptop out and processed a load of photos with Lightroom, but I haven’t done that again.
Obviously if there are passengers then it is much better for everyone, but often one cannot find passengers, and 50% of the potential population (actually way more than 50% if you look at who has the most time) are likely to be problematic one way or another 
Turn the autopilot off and remind yourself that you’ll examine how precise your flying was in briefing offered by software which tracks everything. Can you stay straight and level
In thermals?
One could certainly hand fly long distances (I once hand flew down to Santorini and back, although Justine did help at times; she did the whole trip with me) but it is a tradeoff between staying away (due to the extra concentration) and getting very tired (due to the extra concentration). On balance I prefer to stay on autopilot and have something else to do.
