My sister is taking her husband to San Francisco for a birthday trip. They're both sophisticated travelers but not over-the-top adventurous diners. I'd like to give her two restaurant suggestions for dinners and two for lunches - places that would really create a memorable SF dining experience for them. Iconic, quirky, high-brow, dive-ish - they would be open to all. With so many places in the city from which to choose, this task is a bit daunting for me. Any ideas?