This is a good place to relax and watch airplanes fly, land, and take-off. Helicopters too. Take 101 to Palo Alto and exit Embarcadero. Follow signs to Embarcadero Road and the airport. Head towards the bay, past the airport until it dead-ends. Make a left and drive to the parking lot at the end of the road. From there you'll need to walk on the gravel trail towards, and past, the nature center for about 5 minutes until you get next to the runway. See map below.
You'll need to bring your own portable seating or sit on the ground.