One of the best Valentine's Day activities we've done has been to create a "round robin" valentine for each family member. The kids need to be old enough to read and write to participate, or will need Mom and Dad's help to make it work.
After a nice Valentine's Day dinner, each person gets a blank card and a pencil. At the top of the card, each one writes their name, and passes the card to the right. In just a few sentences, you are to write something special about the person whose card you now have. The card then gets passed to the next person and they write something on the card. By the time the card completes the circle, each family member will have a card that has been written in it by everyone. Have each individual read their card out loud and enjoy the love!
One of the things that is so cool about this is hearing the children express their love for each other. The finished Valentines are real "keepers!" I ran across some from years back and enjoyed reading the misspelled words and funny expressions, frozen in time and oh, so wonderful. It's what Valentine's Day is all about.