I went up hiking at Cataract Falls, which is near the Mt. Tam watershed in Marin. It was spectacular. 2 hours, a very steep climb, and seven waterfalls, with a perfect picnic spot at the far end. It was a great workout because of the elevation gain (which is intense...I recommend trekking poles to protect your knees), yet the pretty falls were a good distraction.
I highly recommend this hike. My favorite has always been the steep ravine and dipsea trails because of the moss and streams, but this one is just as good, if not better!
To get there, drive to downtown Fairfax and turn left (coming from 101) onto Bolinas Drive. Follow Bolinas to the dam and park just over the dam. The trailhead is at the 8.13 mile marker. Enjoy!