The first time she went on vacation (she's been the boss about 2 years now) I drew the outline of a dead M&M on her floor and covered it with a napkin (AKA sheet). I made the eyes out of Xs and took lots of pictures. She loves M&Ms and laughed hysterically when she returned. I had her M&M dispenser on the floor, a few pieces of candy scattered, crime scene tape (isn't the Internet great?), evidence markers, you name it, I did it.
This time, I think I'm going to do the outlines of the five of us in the office. Her office isn't huge, but I think there's room to get five bodies in there. And maybe a missing persons poster of my boss, for being gone so long. That will teach her to take a few days of vacation.