"Feather of lead, bright smoke, cold fire, sick health." Shakespeare had fun with the lovesick, and sometimes even gave them little poems. Hamlet, following the Valentine tradition of dubious verse. wrote to Ophelia: Doubt that the stars are fire; Doubt that the sun doth move; Doubt truth to be a liar; But never doubt I love.
KS3 pupils can write Valentine's messages to and from a selection of characters in their set play. The challenge lies in creating the right "voice" and selecting the right allusions. (The Macbeths' internal post at Glamis Castle would make interesting reading.)