First make sure she likes surprises...lol.
Then, of course it comes down to tastes, what she likes best. A nice dinner you have made yourself, served in a clean, tidy home you took care of :-). Or a a cute poem about her (not necessarily sugary... funny could do it as well), written by hand in a piece of white paper which is wrapped around a rose. How does that sound :-). Went too far..lol?