Monarch has been voted one of San Francisco's top lounges and has a fantastic sound system. It is located 2 Powell St BART stops and the on-street parking is usually easy (though read the parking signs and don't leave anything in your car!). Monarch doesn't serve food, but there are plenty of good, inexpensive places to eat in the area.
This is a 21+ venue, so please bring ID. There will be drink specials with Couchsurfing name tag. Name tags will be provided at the event.