FlyerTalk Forums - Consolidated "What to do during layover/stopover at SFO" thread

The new SFO outdoor observation deck “is located at the far end of the G side (United/Star Alliance) of the airport's international terminal. It will be open daily from 7:00 am until 11:30 pm “as of Wednesday, 6 Feb 2019. “Only passengers using Terminal 3 (United) and the International G-side will have access to it...The 2,997 square foot, roofless deck, offers expansive 180+ degree views of the ramp and runways on the north side of the airport and beyond.” Link to full article and slideshow from sfgate.
~~(TEMPORARILY CLOSED DUE TO COVID)~~ the SFO Sky Terrace is landside (pre-security) in Terminal 2: "... open to the general public seven days a week from 7:00 a.m. – 10:00 p.m. There is no cost to enter and you don’t need a ticket or boarding pass. Visitors will undergo a security check. Food and beverages are allowed but smoking is not permitted."

Outside SFO:

Assuming typical "West Plan" operations, Coyote Point Recreation Area (https://goo.gl/maps/DAUZv7A9bFS2) is an excellent location. There are a few overlooks on the trails by the water, depending what angle you want. Vehicular access is via Coyote Point Dr and costs $6 per vehicle.
For free alternatives consider parking at Ryder Park (https://goo.gl/maps/h48Dk8PhNGk) or Seal Point Park just down the street. There is a flat paved path between here and Coyote Point, although there are overhead power lines along the shore. (Coyote Point and Seal Point parks have unobstructed views however)