The sheer amount of mood whiplash in the game in general. The absurdity of those moments in the sidequests contrasts with the serious plot wonderfully, giving the game its own straight man.
In the karaoke sessions where Kazuma is accompanying, the vocalizations he makes can be gold since they're often yelps or interjections in Engrish... hearing the Dragon of Dojima go "oy oy HEY!" over J-Pop is a wonderful feeling.
Dead Souls's post-credits scene: a drunk man is mistaken for a zombie... right after an entire army of the police and JSDF report.