You know how on your trip there's always "that family" or "that person" that you see or interact with at the start of your trip (on the airport tram, at hotel check-in,e tc) and then you see them EVERYWHERE the rest of the trip?
I've always wanted to go into a park and find a random family that looks like they're the sort to be taking a lot of pics. Follow them around all day, and then try and get myself into the background of as many of their photos as I can. Not necessarily posing for the camera, but make it look like I just randomly was in the background. Wear a bright colored shirt. Then you have to wonder what that family would think when they got home and developed their photos...
-Rob