I need a birthday present for my best friend (she turns 16)
To my girlfriend: She is very creative and her favorite subject is theater (but she has Netflix and often goes to the theater, so I can't give her anything), she plays the piano and likes to sing and do a lot. Besides, she is very sporty.
I would spend about 20 euro, but I could also put together with other girlfriends!
Be creative yourself. Maybe something with a personal story.
Or you write yourself a play and play it to her.
Otherwise maybe something you do together. For example, if you like a roller coaster you could go to a theme park. Time with friends is priceless, and often worth much more than a gift.
Be creative, she's your best friend and you know her best.
I would give her my body for a hot night
PS: then you have to spend no money 😜