We found 3 Reddit comments about Amsterdam: A Brief Life of the City. Here are the top ones, ranked by their Reddit score.
My main source is a Dutch history of the city: "Een Kleine Geschiedenis van Amsterdam" by Geert Mak, translated here. A brief description of the miracle, which is remarkably precisely dated appears here, with more detail on the growth of the tradition of pilgrimage around the site on the Dutch equivalent wiki page and here (In Dutch, unfortunately.)
My apologies for the relatively unremarkable tale, but my first choice (an anecdote on a medieval religious man who took his faith very seriously indeed) evaporated when I was unable to find the book that describes it. This seemed sufficiently odd and remarkable, especially as Mak describes it as a remarkably precisely documented miracle, as there are descriptions of it appearing less than a month after the occurence.
I recommend reading this book and just start walking through Amsterdam. The book kind of opened Amsterdam up for me, and really changed the way I look at it. The city center is small, you can do everything on foot, or take an occasional tram.
I've never done it, but on Wednesdays there are free lunch concerts at the Concertgebouw.
Museums aren't free, but there are plenty of galleries throughout the hole town.