I din't have time to take photos today (couldn't post without so boring so googled this one, playdough tray tomorrow) so am giving you links to my five favourite blogs that suggest Valentines activities for preschoolers (mainly Montessori based), hope you will find then as inspiring as I do.
Valentine's card making basket @ Chasing Cheerios
February shelf activities (montessori) @ Counting Coconuts
Valentine's craft activities @ Crafty Crow
Valentine's activities @ My Montessori Journey
Valentines shelf activities @ Montessori Spanish
Sorry one that I couldn't really pass without drawing your attention to it, it is not strictly a "children's activity" but think you could use with children very successfully.
Heart Page Markers @ It's all Fiddle Fart