Today is my girlfriends birthday, i want to gift her a good gift, she is 20 now

i think you will be the best person to understand what she like and dislike. it also depends on your budget on the gift. If you could afford it, maybe a nice necklace for her to wear..it will be sweet to see her wear it everyday. Or you can handmade something for her if you are creative...hehe..at least i will like that.