Is it feasible to take the BART downtown from the airport, see some sites and then return back to the airport for an evening flight? If so, what would you see? This would be a Sunday. Thanks.
Yes, if you have enough time between the flights. Schedules are at http://www.bart.gov .
What to see depends on what kind of things you like to see.
I agree that it’s OK, especially if you don’t have much luggage because you’ll likely have to carry it with you. Since 9/11, most places no longer have storage lockers for security reasons. We’ve generally found BART to run pretty regularly. Since you’re traveling on a Sunday, you shouldn’t have the commuter crowd but should allow extra time to get back so you don’t miss your flight. 
I agree. Just check BART’s schedule for a Sunday so you give yourself enough time to get back to SFO.
Also, remember that SFO is not actually in San Francisco. It’s at least a half hour away by train. You need to build that into your schedule.
rent a car for the day? May be more pricey, but give you much more flexibility and ability to see things.
I would NOT rent a car for the day. Parking, let alone driving, in SF can be a nightmare for those not familiar. While Sunday parking is free in many areas (IF you can find it), …Fisherman’s Wharf, Embarcadero, areas around ATT Park are not. Additionally, many of the meters have a two hour limit. Green meters = 15 to 30 mins. Rates at meters run $2.5 to $3.5/hour.
Take Bart in to Powell St. = $8.65 ($3.20 for seniors) San Francisco is a very walkable City.
Or Uber.
Do not get a car for a day in SF! Parking is a nightmare, and driving through crowds requires nerves of steel.
I’m a SF resident, and I echo that you DO NOT want to rent a car and drive, especially when BART is so easy.
Take BART to the Powell Street station, and you’re in the perfect location to explore. You’re right at the cable car turnaround. You can window shop around Union Square, or head to Maiden Lane for the boutiques and to view a Frank Lloyd Wright building. It’s an easy walk from there to the gates of Chinatown too,
From there, you can walk to Kearny street and hop on the 8X to get to Fisherman’s Wharf (which might take awhile) or you can walk about 20 minutes east on Market Street or hop back on BART to get to the Embarcadero, where you can stroll along the water and look at the specialty shops in the Ferry Building. The Exploratorium hands-on science museum is on the Embarcadero as well, and you can walk along the water all the way to Pier 39 and its attractions – especially the sea lions.
I’ve lived in San Francisco for 30 years, and I haven’t seen all of the sites. If you’re more specific about what interests you, I can offer more info.
We visit often and usually have a car since we are traveling. But also walk and take BART or Muni. We were there in July and had no trouble finding a spot on the street near the waterfront (around 2 pm) on a Sunday. Getting from the waterfront area to Golden Gate Park or bridge, the beach, the marina, Presidio or other less touristy areas is much easier with a car if time is limited. If OP wants to just stay in the areas mentioned in #8 then of course driving makes no sense, but could to see a wider area. Yes, more expensive. But if it is the only opportunity to see the City, may be worth the cost.
I rented a car at SFO last week. There were long lines at the Enterprise counter and again in the parking garage. It took me 90 minutes to get my car. I’m not sure whether this is typical. But if it is, it’s an issue because the OP doesn’t have 90 minutes to spare.
Returning the rental car was very quick and easy, but I did it at 6 a.m., which is not a particularly busy time. I don’t know what it would be like later in the day.
Great thanks. This is my situation. H and I arrive Tuesday. He has to leave Sunday morning. We will have a car. Our D is meeting us and she and I are leaving together Sunday evening. I just made the return ticket for the evening since you all said it will be doable after we drop off H in the morning.
So now I have to decide if it’s better to return the car early when I drop off H and take BART or keep the car and return it before the 7:30 pm flight. Sounds like a toss up. We can leave our backpacks in the car and I live in a city with traffic, but I really like the idea of the BART to the street mentioned above where we can walk and take the cable car
Car rental at SFO is a crapshoot: it can get super busy. Plus, the rental place is offsite, and to get to the terminal from the rental place, you need to spend up to 20 min on the train, so that time needs to be figured in.
The main issue is whether the OP is able to store/check in the luggage.
If you join Alamo Insider and Budget’s clubs (free) you can often skip the line and get to your car much faster. We got our AAA to get us Hertz Gold for free as well. We’ve given in and signed up for these to minimize waits. The advantage of a car is you can stash your luggage in it. We’ve rented cars for under $50/day or $100 for a week in SF. We usually have a car in SF, depending on where we want to go while there. If we only want to go to downtown & places served by BART, we skip the car.
If you are staying in a hotel, can you just check bags with concierge until you depart?
I’ve done this in several cities (though not SF). But it means going back to the hotel and allowing time for a hotel employee to have a chance to fish around in the storage room and find your bags.
Can you check bags for an evening flight in the morning and just carry what you need for the day in your backpack? The OP and daughter will be at the airport anyway, so checking luggage shouldn’t take long.
One more possibility: Park the rental car in one of the airport lots when you drop off your husband, and store the luggage in the car. Take BART to the city. Come back to the airport by BART. Then return the rental car and take the skytrain to the terminal. This costs money, though, and I’m not sure whether there would be good road signs to direct you from the parking lot to the rental car return.
Car rental is a pain at SFO since it is off site. Can you and your daughter check your bags in early? or
At this site it says:
Can I store luggage at the airport?
Baggage storage is exclusively available at the Airport Travel Agency, located on the Departures/Ticketing Level of the International Terminal, near the entrance to Gates G91-G102. The Airport Travel Agency is open daily from 7:00 a.m. to 11:00 p.m. No reservations are required for baggage storage. All baggage is x-rayed prior to storage. Rates are assessed per each 24 hour period that an article is in storage (articles in storage for less than 24 hours will be subject to the 24 hour rate), and vary according to the size of the object. Please contact the Airport Travel Agency at 650.877.0422 or CF.Airport@gmail.com for additional information.
Be aware that SF has been suffering a rash of automobile burglaries – especially cars that are noticeably rentals and especially near tourist attractions. I would be leery of even leaving possessions in the trunk unless your car is one that permits you to lock/disable the remote opening switch and prevent the rear seat back from being folded down. Actually, I’d be leery even then.
Lakemom’s tip above is your solution, I think (unless you want to deal with the hotel concierge storage). The BART station is in/at the International Terminal also, so you’re right there. The United terminal is an easy walk once you retrieve your stuff; the automated people mover (Air Train) is an easy way to move between terminals if you’re going to a farther one. Just be sure to take it in the right direction [check the maps!] and make sure you don’t get on a Blue line going out to the rental car area rather than one going first to the terminal you want. The Red line is safer for newbies because it just goes around the terminals.
Depends on if the things you might do that day are in places where driving and parking is annoying (but BART is right there) or in places away from BART and other public transit (but driving and parking is not as bad).