I remember how stickers and a blank notebook would occupy my boys for at least 1/2 hour after a meal in a restaurant, allowing the adults time to enjoy our meals. Although my boys have outgrown their sticker stage, I am living in Korea where the stickers don't get any cuter so I find myself picking up stickers for friends with young kids.
Seeing so many cute stickers gave me the idea of creating valentines where you can attach a sheet of stickers. Stickers are great because most kids like them and you can buy different stickers depending on the interests of each child.
Another alternative to the stickers, but sticking with the "Let's Stick Together" theme, is to attach magnets to the valentines. I found a set of magnets at our local shop. My boys used to love playing with magnets and picking up metal toys and paper clips with them.