If she is 15 i am pretty sure that she would love jewellery.It dosent have to be expensive just something from the heart that will show her how much u care.I wouldnt get her lingerie if my bf did that at 15 i would have thought he was a pervert and wanted to see me in it and i would have punched him in the face lol.If you get flowers at least get her one fake one that she can keep forever.My first bf used to do that when i was 16 and at my mums house my room is still full of artificial roses in cylinders with toys that he got me.It still makes me smile when i see them and i always think how sweet that was.I asked him for fake ones because he was always buying me real roses and they just die in a short while and have to be thrown away.Also see if she collects anything and see if you can get her something along those lines.