Reliving a day is totally different to going back and changing something. Without overthinking it (although probably because I've been with my partner for a quite long time now) the days that popped into my head for guilt free reliving would be "first times" with some of my ex girlfriends. When it comes to changing things, there are a number of financial decisions that I would love to be able to revise (wouldn't everyone?) and a few physical confrontations that I would have avoided/handled differently. However if it's just changing something for the day, then a teenage conversation (where I totally failed to say the required words and blew what was turning into a serious relationship) would be interesting to do over.