Don't do it. I almost did once: my ex kept a collection of things about our relationship in a book, a collection of thoughts almost. She gave it to me on our birthday, but then she cheated on me and broke it off a couple of weeks later. I thought at one point that I should give it back, as it wasn't the type of present that meant a lot throughout our relationship and had happy memories, since I hadn't become attached to the book in a happy way yet. I hadn't even read through all of it when it all happened. So to me the book just has unpleasant connotations, reading it just after the breakup, in a way that wasn't intended. However when I mentioned this to her, she was quite emotional. I didn't really think at the time that it might be an emotional thing for her too.
As has been said, if you want to upset your ex, it can be a way to do it. However if you actually care about them being happy, I wouldn't do it.