try to think of 21 places and/or things your friend likes to do and then write each one on a gift card. the idea is that they present one to you when they would like to do it ie. friday treat them to a curry, monday challenge me to a malteser eating competition, tuesday, game of squash etc, it makes the birthday last and last.
If you have known her for a long time, how about a nostalgia scrapbook - with old photos of her, friends, family, pictures of her old schools, homes, what was number one the day she was born. What were the newspaper headlines, prime minister etc. What were the most popular shops etc. and fashion.