As a woman who has a great fear of heights, I would pick being pushed out of a plane with a thin piece of silk to keep me from plummeting to my doom over this:
Or maybe I am just funny that way.
Just a card is a bit crap really, isn't it?
It really depends what the budget is.. but hey, here's one.
Assuming good weather, take her out in the car (some bullshit reason to go somewhere). 'Break down' at a certain place, where a short walk away, marked out with ribbons (visible from the car and on a tree/bush every few feet), is a picnic hamper, blanket, champagne, etc. For added effect, tie some balloons to the hamper, with helium of course.
Max cost - with lavish food. Strawberries and the like.. about £50/$50? Unless you want to go crazy with the champagne.
Assuming it's a good choice, jewelery would be a good addition. If you ask one of her friends to help you before hand, the friend might leak the info or will probably make some comments about you that'll make your gf think, but hopefully the friend will know your gf's taste. If your gf suspects something good in the next few days, she'll enjoy it more when it happens. Generally, of course.