My 16th cake was very special and I really wanted a 'grown up' cake so that's what I got. A shop in a nearby town bought in Raymond Blanc cakes and so we ordered a fresh strawberries and cream one. It looked amazing; really pretty and very sophisticated for a 16 yr old girl.
Going with the laptop idea, kids cakes based on the DS, have a cake base and keyboard with a cardboard back piece, made to blend with the keyboard. Sounds tricky to make though. If she doesn't mind you not making it, you could buy her a really pretty special occasion cake.
I think a laptop may be a bit tricky, if you're a bit of a novice (as am I!). How about making a big square cake covered in fondant icing then wrapped with a huge bow to look like a parcel? White icing and a huge black bow could look quite stylish and fashiony in a Coco Chanel or Mary Quant kind of way maybe?!
Hmm, thinking about my 16 yr old niece I don't think she'd really like a novelty cake as it's not "cool" (at that precise age, I mean, when they're trying to feel grown up. I think they become cool again when they get to university!) obviously you know your sister best, though.