Here is my idea of a good joke for valentines day:

Tell your lover that the thing you want most on valentines day is a good old fashioned chocolate sauce rim job. Say you will be heartbroken and die if this wish is not met.

Allow them to drizzle chocolate liberally accross your rectum. When they're putting the cap back on the bottle of sauce, poop a little bit. Just getting a little head of poop to stick out will be sufficient, but if you have mild diareaha, all the better.

They will eat poop without even realizing it!!!

A word of caution: Do not kiss them afterwards, asshole.