I have two suggestions.
One thing I did for my daughter who is now 20 is I made her a special CD of songs I purchased online and downloaded. They were all birthday songs "Happy Birthday Darlin" and "Its my party and I'll cry if I want to" and "16 Candles". But also, "My Girl" and "Brown-Eyed Girl". There was a mix of funny and serious, and she still treasures that CD. Sometimes she plays it in the car when we're traveling and we have fun singing along with all of the songs. We played the CD for the first time during the party, and its still a special keepsake.
Two: if you look on wedding sights for caketoppers, you can find some really pretty crystal or metal caketoppers that say "16". Make or have made a special pretty cake, in the style of a small wedding cake with pretty and pink and elegant styling with the 16 caketopper on it. It will be a showpiece and something that she'll treasure.