Here's one of my favorite valentines: a mechanical change-a-face card, dating to the 1930s. By pulling the tab at the bottom, the goofy-looking boy's expression changes. Just a really fun valentine!
Here's a great set of 1907 sequence postcards: these were mailed by the sender one by one, with the complete set telling a little story. These postcards were sent by a woman in Washington state who traded similar sequenced sets with a friend in Michigan. The message on the back of the first card reads: "Kind friend, yours received yesterday, couldn't get the rest of that set, therefore I shall finish with this." Pretty cool that these were kept together and saved for over 100 years!
This is one of the most unique valentines in my collection. Made by Raphael Tuck in the early 1900s, it features a die-cut Dutch girl overlooking a rebus constructed of gilt paper utensils. Unusual and attractive, it's quite a treasure.