Now THAT is a loaded question.
I have to assume I would really only have the 24 hours and that any travel and the likes would be included, so I would not waste a moment on this.
My first idea was to just simply muck around with people's minds. Go to a shop and start moving stuff around. Let them see things float in mid air or move from one place to the next. I am sure I would get bored of that after the first 30 or so minutes but the reactions would be funny. The best bit is that no-one will be harmed, although watching them panic could be a laugh.
I might try and spy on the guys at work and see what they really say when I am not there. Strange one that as I am not 100% sure I would want to know to be fair. Better the devil.....but there could be some fun chats I would not mind listening in on between some of the girls in the office. What do they talk about?
Of course, there is the obvious one of being in on a girl when she is alone to herself. There is one girl I know who I would love to watch and see what she gets up to of an evening. Whether it be simply watching the TV or getting herself off, I would just love to admire her up close without her knowing. She has no idea how much I adore her and it's a shame but a reality. She probably doesn't know I even exist, but the ability to discretely spend time with her and knowing what makes her tick would be a massive bonus.
Boring in the end, I guess, but what would you do?