Who says you need to be serious on a Valentine Day! You can send funny cards and stuffs to your valentine on the special day. I found this little mathematical equation for you graphing calculator.
r = a(1 - cos(theta)) =>Plot that on your TI-89.
That was mitadmissions.org/topics/life/student_life_culture/my_funny_valentine.shtml" target="_blank bryan's idea. He has a nice and funny Valentine card in the page too:
Got this nice drawing in email : Always be prepared for the worst scenario when you give your heart away (I wish the art were not gender specific though).