What to wear at a three star restaurant?

My boyfriend is taking me out to a nice 4 star restaurant, but he told me to dress as if I was going to a 3 star one. He said a nice dress but nothing special. I still don't really know what to wear. This is the first boyfriend that actually takes me out to places! And the actual restaurant is a surprise, so I don't know where I'm going. Can I have some help? Pictures would be nice. What to wear...