A world where, instead of aging in years, the people age in knowledge. So, instead of dying once a general age was reached, they die once they achieve a certain amount of knowledge. A man could live forever if he never learned anything, or could die after reading a few choice books. Of course, the knowledge of the land would be regulated, like medical drugs.
A healing potion which if drank by males (or women in male clothing) causes a short term (1 d6 hours) urge to dress in girl's clothing due to the high levels of estrogen in it.