As the author of this post, I can tell you that many of the things I do day to day are not mentally challenging, but are still fun. Have you ever played any games in your life? Are they still fun when they aren't mentally challenging? Of course. If you enjoy something, then it will be fun whenever you do it. Not just cool problems or challenging ones, but all problems. The only part of my job that isn't fun is going to pointless meetings..but that is a crappy part of most corporate jobs.