You should throw her a surprise 21st b'day party..invite all her friends.
gift? a bouquet of flower is a must for any girl.
i suggest something expensive because 21st is so called the big thang.
buy her a gold necklace? or maybe white gold will do with a cute pendant.
other option is you can take her out for a expensive candle lights dinner and have everything planned out with the waiter on how you want them to present the cake, flowers and gift.

buget 1k is good enuff.