Since you have a good relationship with your cousin, you must know what kind of girl she is. If she is a cute girl, you can usually give her some plush dolls, mobile phone pendants, hairpins, cute notebooks, etc. If she is more mature, you can give her necklaces, roses, rings, diamonds, etc.
For a birthday like 20, you don’t have to be so formal. People at this age don’t like etiquette very much, so it is recommended to give a fashionable gift, such as a new MP3, or a pair of cute small speakers. There are those DIY portable personalized speakers, which are very interesting.
Recommended name necklaces, makeup mirrors, blessing chocolates, cartoon bouquets, all of which are creative gifts. There are all kinds of them, and there are other gifts. I hope it will be helpful to you.