If it's not for a special occasion, be thoughtful. Dinner, as mentioned earlier, is a very good way to go. Plan out an entire night, set it aside for both of you, no cell phones, things of that branch. If you don't want it to be too serious, do something entertaining after dinner that's not necessarily esoteric, but not trite (movie etc). The memory of an enjoyable experience is often times vastly more affecting than a mere gift.
Unless she is a golddigger, she'll appreciate anything that you put a lot of thought and effort into. Anyone can go out and buy jewelry; unless she specifically eyed a certain piece, I wouldn't go that route.