Whenever I go on a trip, my son sneaks a stuffed animal into my suitcase. I then take pictures of said animal around the hotel/conference center/tourist spot.
I could say that this started when my son was very very young--maybe even that it was his beloved toy coming along. But no, this just started a few years ago--if he wasn't a teen already, he was definitely a tween.
The first time it was a surprise that he had done such a thing, and I was touched. Then he did it again and it became something of a tradition. Now I make sure to mention that I have finished packing, just in case he wants to sneak something in.
I found Mr. Turtle (pictured above) before I left home this trip--I considered changing him out, as he's already been on a trip to D.C. with me AND he is not small or squishy enough to slip inside a purse or pocket. But that didn't seem fair to my son, who had already left for school.
Maybe I'll make a maximum dimensions statement, just like the airlines...