Something we do with our son is "The 7 days of Valentines". Not as popular or well known as the "12 days of Christmas", but fun. We do it with clues and special little trinkets, treats etc each day. Doesn't have to be expensive, but fun little creative things - builds up to one nice valentines present. This year it started with a little valentines card setting the stage for 7 days of fun and mystery and it just had a small package of sweet tarts in it. A couple other days are skittles and other little candy treats along with coupon for a couple fun things as a family, a bike ride with Mom and one for a driving range day with Dad. Then a package of Webkins trading cards the next day with clue card in the morning ($2.95 for the cards I think), an I-Tunes music card for his I-POD ($15) and we are ending on valentines with a Webkin stuffed animal.
At the age of 15, if your daughter doesn't have a special ring from you - a nice one from Mom & Dad to remind them the love that is always there and never ending. (hard because the 6 year old may feel left out, but it means different going to a 6 year old than a 15 y/o typically)
Also, from growing up one of the best memories I have is that my father always brought my Mom, my sister and I boxed chocolates and flowers (for "his girls") Remember the laced trimmed boxes of brachs chocolate? now my palet for Chocolate has matured but at that age - it was such a special treat. (I think it also set the tone for the way I felt I wanted my boyfriends and later my husband to treat me - special)