If it's possible, take her away for a surprise overnight stay somewhere. Take her for a romantic meal. Give her a shopping voucher for her favourite store. What are her interests? Does she like the theatre- maybe take her to a show.
The most important thing for me is the card, it has to be very meaningful, and you must write your deepest feelings in it, so much so that it brings a tear to her eye, then like the other answer someone said present her with an eternity ring and she will love you forever!! (and not nag for at least a week!)
I agree with Beejaybee, a pamper day. The Hilton hotels do a spa day and you get 3 treatments and lunch thrown in, I got it for my 30th Birthday of my parents and loved every minute of it. They also have a nail/hair salon too so you she could get pampered to her hearts content.
I'm trying to organise a pamper day, thanks all, but she will need to take someone with her (not me). So I've suggested she invites her sister and I will subsidise her sisters day.