The Senior Clipper Card is a good deal. You need to be 65+ and fill out an application with proof of age to get one, but there is no fee. We downloaded and filled out the form ahead of time, then got our cards after a short wait at the Transit Store at the Powell BART stop; there are eight kiosks at various BART stations. You can pay the clerk cash or use the credit card machines nearby to add fare before using.
Once you have a Clipper Card you can tap and go on any transit, including the cable car. On our first cable car ride, the conductor saw our passes but didn't use the handheld scanner, so we rode for free!