Easy origami heart
This is a straightforward fold that makes a really neat little paper heart. Either print out these instructions from www.supercoloring.com and let your students tackle it independently, or use one of the many videos on Youtube. This one is particularly clearly demonstrated.
Students can open up their hearts and write messages on the inside. Alternatively you can spread the love around by doing some 'origami bombing'. This goes down a treat!
Drawing a Cardioid
Resources provided: a presentation which introduces the cardioid, shows examples in different contexts, and demonstrates the drawing procedure; printable template. Resources needed: pencils, rulers, erasers, coloured pencils or pens (optional).
Plotting parametric hearts
Older students can practise plotting parametric or polar functions to produce a heart-shaped graph. This can then be cut out and stuck onto card to make a truly geeky Valentine's card :) There is a good selection to choose from on Wolfram Mathworld and some alternatives at Owlcation. And this one from 9gag uses the modulus function: