Being aware that this thread is already a couple of days old, still wanted to add some info about renting a Nissan Leaf.
A year ago, when I contacted Enterprise about their Nissan Leafs for rent in California, all individual locations that I called told me that they do not have Nissan Leafs anymore. Also in Northampton, MA, Enterprise does not have NIssan Leafs anymore. The last market I still saw Leafs in, was in Orlando, Florida.
And looking at the Zipcar website right now, also for the Bay Area market it does not show any Nissan Leafs when using the "Search by car type" functionality.
So there are few options left, RelayRides being one of them, if one would like to try it though the insurance situation surrounding these peer-to-peer rentals still contains some controversy, and also the convenience factor might be different than picking up a car from a regular airport rental car counter.
If another electric car would do, and it doesn't have to be a Nissan Leaf, BMW operates its "DriveNow" scheme in San Francisco, f.e. there are several BMW Active E and i3 over at the SkyPark in San Bruno, not far away from SFO airport.
If it's no problem to take BART from the airport to downtown San Francisco first, and it's no problem to enrol in a non-profit car share program requiring a monthly fee (DriveNow does not), then CityCarShare www.citycarshare.org
might be worth trying. At their Sutter & Divisadero location, they also have Nissan Leafs for rent (besides Ford Focus EVs, and Scion iQ EVs), and $48/day plus 10¢/mi seems pretty inexpensive.
One could also try to contact Hertz, as they used the have a car group called J class Volt/Leaf/Spark at SFO ariport. As far as I know, that electric car class is also permanently "sold out" aka not available anymore. I often ended up renting a Prius from Hertz. Of course a true electric like a Nissan Leaf is much better, still a Prius at least is somewhat more environmentally friendly than a non-hybrid ICE car.