Despite being less than 50 sq. miles, San Francisco is truly a mecca for foodies. If you have the financial means to do so, I would recommend spending one of your visitation days with an organized walking food tour of "The City". One is www.foodtoursofsanfrancisco.com
and they currently offer food tours of North Beach or the Mission District. They're also planning a tour of Chinatown but it is not yet available as of this writing. Tours are $60 per person per tour. North Beach is well known for its Italian fare and the Mission is known for its Mexican/Latin American cuisine. Yes, the HUGE Mission burrito originated in the Mission District. Another company that offers walking food tours is "Local Tastes of the City"
. They also offer tours of North Beach and Chinatown as well. Tours are $59 per person per tour.
There are other companies offering food tours as well. Just Google them. Of course, if you've been to San Francisco before and you feel you know your way around the city as well as the natives then go on your own. If this is your first time (or even your second or third time) to S.F., you will find how easy it is to get lost even with a city map. Going by car is okay through most of the districts outside of downtown, but, my gosh, trying to drive through Union Square, Chinatown, SOMA, and the Financial District with its small, one-way streets can be a total nightmare especially during "rush hour". Best wishes on your stay in San Francisco. Hope you have fun!