This isn't a huge elaborate activity, but has been a hit in my house for a few years. We just have a plastic Valentine's box that has an easy to remove lid. We cut (or you could use a scrapbook heart punch) a bunch of paper hearts of different sizes, and the whole activity is to put all the hearts into the Valentine's box. I know it sounds so silly, but my girls love to do this over and over again and have for the last couple years too. They like taking the lid off and starting all over again. You can have them take turns or have multiple different colored hearts and make them sort the hearts by color and only put the pink ones in the box, etc. It is good fine motor skills practice for little, little kids to place them in the little hole at the top. My kids love putting money in their piggy banks, and this is the same idea, but just Valentine's related. Plus it is a cheap activity and it doesn't really matter if it gets crumpled up.