Haha, what a coincidence! I am also born in the year of the monkey, and my preferences may be similar to your girlfriend's. Then I will tell you the gifts I like, and you can refer to them for yourself. Women's agate bracelets, fashionable hollow makeup mirrors, cartoon bouquets, these are all very popular with girls.
If your zodiac sign is monkey, you can give her a big plush toy. If you think it is too ordinary and want to be more creative, you can consider giving her a Yo-Hoo Monkey. Imagine her reaction when she gets the Yo-Hoo Monkey. She will never think that you would give her a gift of her zodiac sign, and the Yo-Hoo Monkey is so funny that she will definitely be very happy.
How about a beautiful sun jar? You can also hand-paint the appearance of the sun jar yourself, and draw your thoughts for her, or make your couple photos on it. Such a scene must be very warm, and she will definitely be very touched. In fact, your thoughts for her are the most precious to her.