I'm in SF for the week on vacation, near Nob Hill. I'm going nuts without riding! A buddy of mine and myself are looking for a good ride or two during the week. Any suggestions? We were thinking about mt Tam, but don't really know where to begin. Any suggestions about riding near the Presidio or just across the bridge? We were thinking more along the lines of XC riding, but a good dh run or two would be fun, depending on what we find for bikes. Any suggestions at all would be appreciated.