A.C.
Is this something you want to do, or you want your family to do? Because my first thought was if it's you wanting to recognize this, (in addition to whatever family day activities), how about asking her to help you plan a day trip/day of fun activities for the 2 of you. Could be primping/nails done/girly stuff, could be taking her ice skating just the 2 of you & lunch after, could be renting her fav movie and make sundaes together to eat during the movie, etc. At at the end, when you go in to say goodnight, give her the letter to read. (I love the letter idea!!!!! :) As part of the day, during one of the meals, you could just let her know how proud of the person she is turning into and how she's growing (keep most of the it in the letter, so as not to spoil it)--hearing it out loud I think is just as important as having it written down.