I think it's nice to recognize special occasions with something other than a gift card. Maybe it's just me, but once the card is "spent", it's gone. Maybe she'll buy something lasting/memorable with it, maybe she won't.
You don't say how much you want to spend. That would help. Do you know her well, or mostly are you relying on the fact that she is special to your son? That would determine how personal the gift is.
Have you been in her apartment or house? If she has houseplants, for example,how about a nice decorative ceramic pot (with or without a plant in it)? It's something to display, but it's not high personal (and since you don't know her all that well, you maybe can't go "personal"). One thing we've done is given people 30 lottery tickets - I'm not a lottery player myself, but some people just adore it. I've also given a small wine rack (like 6 bottles) and an assortment of wine to go in it. I knew the recipient liked wine but not what type, so I got the wine department manager to help me with 6 bottles of varying types. I told him my overall budget and he helped me pick out some things - some were $9 and some were $19, you know? But it was a nice thing, and I put the bottles in the rack and tied it up with ribbons to hold them in place. But this only works if you are actually going to the party and can take the gift - it's hard to ship! It does look nice at a party when people are opening gifts though.
But I agree with the others that you should ask your son - not necessarily on what to give, but more about what she likes to do. "Shopping" is kind of vague so maybe he can help you more than that.
If he can't help you, I'd go into a local gift shop with unique things and tell them your problem and what you know about the girlfriend. Maybe they can help you choose something, or help you put together a gift basket of several fun things. Those places often make up something very attractive to look at, and if you don't know much about the woman, then an assortment of things guarantees that you will have at least a few things she loves!