Do nothing. Of all the holidays, this would be the one to do nothing. Let it go... but be prepared that it won't be as much a "shock" to her as you think it will be.
I took my stepkids and son to a pottery shop. My original intention was to have them make something for my W for V-Day. But as I thought it over more I changed it and had them make something for their rooms at the new house. So they could have something of their own that we made together.
SD decided to make something to fit the bathroom decorating theme in the rental house. So I guess it's sort of a gift for W from her, but that was her choice, not mine.
Married 6 together 8 Me:38 W:31 second marriage for both SS12, SD10, S6 Bomb: 9/8/11 (day before our 5 yr ann) W moved out: 2/18/12 D final: 11/12/12 Share S 50/50. Spend as much time as I can with SS & SD