Somthing that she will like...from the heart and something meaningful to her....your her boyfriend for goodness sake, you will find the right thing...
A car.
It really depends on what your girlfriend likes. Earrings? Music?
I think best gift would be piece of jewelry.
Something Unique and out of the Ordinary that you know she would like.
Make her a card, and show her love.
I recommend a handcrafted wooden jewelry box. It is not too personal but definitely shows that you care especially if she likes jewelry. Maybe she can keep some other trinket in it.
Since it's your girlfriend, you could send her one dozen long stem roses. This gift would be a romantic, thoughtful floral gift for her.
If you want a unique gift for your girlfriends 21st birthday, you can go to www.ugiftideas.com/girlfriend. This offers around 100 ideas to choose from, so you will most likely find the perfect one of a kind memorable gift.
You and your family
Unless your girlfriend is an adult and/or your fiance, this sort of gift may be too "personal" a gift for you to give her. Her parents my consider this inappropriate.
candles
a gift card for a cheep pedicure........