Most public transit networks I have used offer a so-called day pass, weekly pass or similar ticket that allow unlimited rides during a limited time, on the whole system or on certain zones in case of wider service areas.
I was not able to find any information about the existence of such a pass on BART. The best deal offered is a high-value ticket that contains more dollars than it actually costs. Getting it is rather inconvenient since it should be requested in the mail, making it unsuitable for tourism. Does it mean I need to pay for every single trip even if I need to heavily use the system for a few days?