The farmers market is only on Sat I believe. But the Ferry building is pretty kewl anyway. I worked on Union Square and would get off at the Embacadero on nice days for a little excercise. I think walking Embarcadero to Pier 39 is longer than Embarc to Union Sqare but I'd have to check a map. There are plenty of taxi's around if you poop out! Chinatown from Grant street up is kinda steep if you have walking issues. There are lots of maps posted about. It's a pretty friendly tourist city. Have fun!
Claudia