Just buy something she can remember you by. Depends on what she likes and her age, but keep far off from any sexual gifts. A birthday is a memorable day for any girl, so it doesn't have to be something that just reminds her of your dick. Choose something sentimental that will make her remember the day and you both, in her own special way.
Most girls would love to have an article of clothing or accessory. If you can spend, try a branded garment, shoes, makeup set or something similar. Jewelry is not a good option in my opinion. It reminds her only about the expense and not about what it expresses. Always make sure that it's not something that can be worn everyday (lest you make it less special), but can be worn occasionally. Something too expensive and unique will keep gathering dust like a museum piece and will be worthless as a real item of interest.
You can also buy her something truly useful and perhaps she can't afford to spend for. Again, this is something that will heavily depend on the girl. As an example, some ways back I had a girlfriend in a university and my first birthday gift to her was a rechargeable fan with a light. Her boarding life massively improved because of it, and she used it throughout her university life thereafter, and still keeps it at home.
She had its batteries changed like 6 times because she was so in love with it and never wanted a better model or any other fan. Something truly useful like that really pulls the heartstrings of a girl. If you take some time to think about your girl, you'll probably be able to identify something like that she would really love to have.
If this is an older girl, (like 27 or above), you could focus more on the experience rather than a physical gift. Girls at this stage of life can probably buy whatever gift you want to give with her own money, so the value isn't really the point. Take her for a memorable day out, trip or activity, and make it a day she can remember. Give her something small and personalized as a gift on the moment. That should be enough, as long as the experience makes her cherish the day she was born.
I hope now you'll have a better idea on how to make your girl's birthday memorable now. It truly has to come from you, but I hope this made it easier.
