It must be something that she doesn’t have, something that you’re certain she will use/wear/work on, something not as cheap as a can of soda and definitly not the classic flowers and box of chocolates, those have to be given in a regular basis anyway.

If you feel like giving her something “special”, then you need not to buy a thing, just make her a great dinner, sing her a song or make a home made portrait, something cheesy, I know, but that she will appreciate.