When it was my best friends 21st i had the same problem. All the 'good ideas' for presents had already been taken.
What i did was buy her 21 different presents ranging from special things she could keep, like jewellry, to very simple things like her favourite sweets - a couple of packets of Black Jacks.
Also include things like a nice bottle of wine, some pjamas, a scarf or maybe some really nice hair care products and not forgetting some perfume.
Hope this helps, you can spend as much or as little as you can afford.
Oh and by the way my friend loved the present, i wrapped them all seperately and then put them in a nice big gift bag!!