If you have a family heorloom to hand on, that would be nice. But failing that I would suggest a memory. A fr<x>amed family portrait or something similar. But its the time and occasion that you create that will matter most in the long run.
Give her the 100 dollars so she can get what she wants
Give her the $100, tell her you trust her. Tell her you love her. Then leave her alone.
a chastity belt and you keep the key.
Give her something that will last as long as she lives. If this was a gift for a niece or a nephew then you could get away with giving them money or something equally insignificant, but this is your daughter. A 21 year old that is probably ready to go on and start a life of her own. Give her a piece of jewelry or something of value. It doesn't need to be expensive, it just needs ot have meaning, Give her something that will always remind her of you as she moves on, and then make sure to tell her why you're giving that to her. <br />
Trust me. That's the best gift you could possibly give someone. I've lived through sixteen birthdays and never has my father given me something like that. I wish he just knew.
Give her the money and let her decide...this was the best answer for my kids after they became teens and still to this day.
The best idea is the give her the money - that way she can get what she wants :)