This is a quick drive from San Francisco and a great place to take in the sunset next to a beach bonfire. You can bring your dog (on-leash) so it doesn't get much better.
Take in a scenic sunset and enjoy the warmth of a bonfire on the beach! Muir Beach changes the number of fire pits it has seasonally. May through November, the beach offers 6 fire pits, and in the colder months (December through April), there are 3 fire pits on the beach toward the south end of the parking lot.
This spot is a little further drive for city folk than Ocean Beach, but on the plus side, you'll probably be beating the crowds in SF. Keep in mind that the beach closes 1 hour after sunset. Bottom line, if you're looking for an isolated spot to take in the sunset, build a bonfire, and hang out with some good friends, then this could be it! For additional information on the area, head to the Golden Gate National Recreation Area page.
If you're looking to pick up some last minute supplies, you can head to the Safeway in Mill Valley.