Prep: cutting out and writing on cards.
This is super super simple and a little too late for this year's Valentine's Day, but a nice quick warmup in case you have any Valentine's classes tomorrow!
However many students you have, cut out half as many hearts on colored paper. It helps if they're pretty big! Choose some conversation hearts-esque phrases on their level of English (for 5th graders, I used "Happy Valentine's Day", "You are kind", "You are cool", "Hello friend", and "XOXO"), and write one on each heart. (I used a few different colors of marker to avoid making multiple cards with the same color paper, color marker, and message.) Then, cut the hearts in half broken-heart style, and give one half to each student.
Start by explaining the difference between Japanese Valentine's Day and Valentine's Day abroad (or El Dia de Amor y Amistad), and introducing the conversation hearts candy. Give each student half a heart, and explain that they're looking for their match. Have them walk around the room having a short conversation with each other, and then comparing hearts. For the conversation, I used a pretty standard rock-paper-scissors where the winner asks "What (anything) do you like?", but anything goes. If it's a match, the students try to read the message and figure out what it means.
You can play it until everyone finds their match, or set a timer and see how many students can. Afterwards, review the messages. Again, just reskinning a classic, but I wanted to help buff up the Valentine's Day tag!