First BART gave away one way tickets valid for weekend usage, now it's MUNI's turn.
MUNI is doing a $2 Sunday FunDay Pass. You just pay $2, and you get a transfer valid for the entire day.
If you use a Clipper Card, the instructions mentions you just swipe your card on the 1st ride of the day and then just flash it at the driver after that. It mentions not to swipe your Clipper Card on your 2nd ride of the day and on rides after that because the system will charge you for those rides, and no refunds will be issued.
Promo is not valid on Cable Cars and not valid on Candlestick Park Express buses.
For more info., click here.