Either ways she's going to mulct you... and whatever you give she is not going to be satisfied... Why worry and brood over the fact then?
For an idea of presenting it, I would suggest, you hide something valuable in say a vending machine or so... and accidentally pass over and ask if she wants a chocolate and then instead of taking the chocolate take the gift and present it to her... As for gift, I leave it to your imagination, creativity, and ur GF.
for she should have hinted you already a million times what she wants...
PS: if her hint doesnt fit in the vending machine.. pardon me!