I think it should be something spontaneous and out of the ordinary. Going to a classy restaurant is so cliche. Why not come up with something unique that she will remember for the rest of her life? Walk through a field of flowers and pick her some flowers. Watch a sunset together while having a picnic and some good music playing in the background. Be original and think of stuff you can do that won't be expected.
I will do my best to reply with an educated, humble and honest answer. Ultimately, it is up to you whether you want to listen to my advice or ignore it completely. Sometimes, my advice may be wrong; occasionally, it will be right. Regardless, I want to do my best to give people answers they are seeking.