When a girl turns 20 years old, it signifies that she is entering the third decade of her life. This is a symbolic milestone, even though she is not awarded any additional privileges in American society like on an 18th or 21st birthday. Still, there are plenty of gifts to celebrate and set the 20th birthday apart from others.
Since she is turning twenty, consider celebrating it by either buying 20 small gifts for the birthday girl or by buying her a set of 20 something, such as a bouquet of 20 flowers or a gift card loaded with $20 or one with a sum in a multiple of twenty. If you decide to opt for the gift of 20 smaller items, consider making the last one a bigger ticket item than the rest.
Gifts to each represent one of the two decades that she has lived through along with one to represent the new decade that she is entering are a good idea. Consider theming them to the different periods of time. For instance, if as a little girl in the first ten years of her life she loved chocolate, make the first gift a box of chocolates. The third gift of the set should be something to help her in her future, like something to help her with a new house, a new job or a new endeavor.
It's hard to go wrong with buying a young woman a nice piece of technology that she will put to use often, such as a laptop, a new MP3 player or a television to go into her dorm room or apartment. Make sure to find out what her electronics wants and needs are before you buy. For instance, if she is in graphic design, she may appreciate a tablet computer or a copy of the newest edition of one of the Adobe graphic design software programs.