I usually try to give people something they need. Is this her first apartment? (Then she probably needs lots of things.) If not, is there anything you noticed her doing without, at her last place?
It depends on her. Does she like coffee? Does she have a coffee maker? Does she need a blender? Does she have enough silverware/plates/things you need if you want to have people over for dinner?
But you're her boyfriend, so maybe you want to get her something more sentimental.
Do you have a sense of her style? Like, if you wanted to get something decorative, do you think you could pick out something she would like?
I remember when you posted some quotes here from her facebook page. She seems like she's crazy about you, so probably anything you give her that will remind her of you will make her happy
Nice fluffy comforter for her bed? Nice bath towels with matching wash cloths? (maybe put that together with some fancy soap or bath salts?)
Or a piece of artwork for her new walls?
Will you buy her lots of flowers in her new apartment? Maybe she needs a vase
Good luck deciding!