Oopsey daisy blog has an interactive valentine that lasts 10 days. Each night the recipient scratches off a new heart to see the message. It's fairly complicated and not for young kids to make but they will enjoy receiving them.
Good for ages 11 and up.
Get the instructions here:
http://www.oopseydaisyblog.com/2012/02/scratch-off-card-easy-peasy-valentine.html
Creative carmella makes one big scratch off field.
Get the instructions here:
http://www.creativecarmella.com/2012/01/i-liked-it-so-i-put-pin-on-it.html