I was motivated to write this post whilst reminiscing about my campaign that recently ended. I was recalling how much I enjoyed running Tegel Manor updated for 4e. I have posted before about the good times to be had with funhouse-style alternate rewards, but I felt the need to emphasize how great non-mechanical conditions can be.
There are endless possibilities for this sort of thing: a player could become itchy, thirsty, starving, frightened, romantic, violent, paranoid, dim-witted, narcissistic, or simply think they are a chicken. And on and on. Any extreme/exaggerated characteristic will work wonderfully. Don't make assumptions about how your group might react to something like this; a little silliness has an odd way of engaging players!