You know what's the greatest plot device ever? The body switch. Two characters, typically with very different personalities, switch bodies. Hilarity ensues. I defy you to watch a movie or TV episode involving a body switch and not be mesmerized.
At the moment, I'm watching an old episode of Angel. Angel has, against his will, switched bodies with some old guy who doesn't know Angel is a vampire. It's awesome. The old-guy-as-Angel is having totally inappropriate interactions with Angel's friends and acquaintances--hilarious! And there is drama too--what if the old-guy-as-Angel goes out in the sun, and Angel dies? How will Angel get his body back? I love it.
While TV can do a good body switch story, the best examples are in film. Here are my top five:
1. Freaky Friday (Jodie Foster version)
2. Vice Versa (starring Fred Savage and the brilliant Judge Reinhold)
3. Freaky Friday (Lindsey Lohan version)
4. Big (not quite a "switching" movie, but the same idea)
5. Like Father, Like Son (starting Kirk Cameron)
Other suggestions are welcome.