If you are just planning on doing San Francisco, then you should be fine, and San Francisco is pretty easy to navigate, if you are lost, google maps is your best friend to get around. One thing I would do is go on a
Big Bus Tour in San Francisco, at least for the first 2 days, sight see around the city.
Again, if you want to go elsewhere, then it might be too short. Consider heading up to Sacramento to see the Christmas lights, or Napa Valley for wine tasting or coffee.
Kind of depends on your budget. But I'd say set aside at least $60 or more just for meals, which equates to $20 per meal. Should be enough to get you through the day.
A few breakfast places to look out for would be
1.
Mama's
2.
Blackwood
Some other things to look out for would be the sour dough bread bowls, either filled with clam chowder, or chilli
You should stay clear of Pier 39, huge tourist trap, beautiful place to take photos, just don't spend more than $30 on souvenirs there.
AirBnB is pretty safe as far as I've stayed in them. If you wish to have a more immersive experience, you might want to try
couchsurfing.