Maybe one of those past, present, and future diaond necklaces but, only with a "present" diamond. You can tell her she'll get the other ones as soon as you actually have a past or if you ever foresee having a future. Thats sure to not set the bar too high and leave room for future gifts. You can thank me later.
But, I would agree with the card and dinner idea. You cant really go wrong with that.