My new go to spot when I am there during the day, is on S. Zeyn Street, in front of the Worldmark Hotel. They have open street parking there, with the only restriction being No Parking between 9:00 PM & 4:00 AM. From there, it's an easy 1.1 Mile, 15 minute walk to the esplanade. Or you could take the ART bus from the Worldmark for $6 with a Day Pass, or Lyft tends to run around $6-$8 each way.
I've had the Flex Pass since last May, and have not paid for parking once, even though I've gone to the parks countless times. It makes it so much better on days like today, when I had to leave for an appointment after spending less than 4 hours at the parks this morning. I consider parking to be the biggest ripoff, especially because you're not even parking right by the entrance. You have to wait for the tram, and then it takes several minutes for it to travel to the entrance. It almost feels like there's almost as much walking involved with parking in the lot, and walking to the trams, as I have walking the 1.1 miles from my car.