A selection of cooler-than-store-bought-anyway printable DIY Valentine's Day Cards.
Minimalist Vader and Magneto Valentines by Anthony Petrie - PDF's here
Katie Cook for StarWars.com - PDF here
Grant Gould for StarWars.com - PDF here
JustJenn's Pencil or Lollipop Holder Valentines - instructions & PDF's here
Mark Anderson @ Andertoons has printable sized scans of an whole vintage book of DC Valentines.
I love these ones with the action features but there are basic ones as well at his site.