If I really do not want to think at all I tend to read Sanderson, Neal Stephenson, maybe a re-read of Douglas Adams, light historical fiction like The Sunne in Splendour, erotica, books on the occult or superstitions, Hunter S. Thompson, Bret Easton Ellis, Dan Brown. I can also enjoy certain types of "conspiracy theory" books as long as I don't believe them too easily.

Usually, though, I consider that "fun reading" is something extremely interesting and engaging but perhaps somewhat complicated, like Biblical criticism (usually textual criticism using the original languages), Buddhist philosophy (I thought that Nagarjuna's Mulamadhyamakakarika was fantastic and no, I did not need to check the spelling on that one), a structural analysis of ancient Egyptian poetry or discussions on the development of the Greek language. The book about ancient astronomy that I reviewed last year was another such book. All of those books were read at a time when I was looking for a "diversion" after having read something particularly heavy (like Gibbon's Decline and Fall of the Roman Empire ).