The trail we took is called Newton Canyon. There were supposed to be some falls, but this isn't the hike to go on if you want to see some falls. You can kind of hike to the top of the fall, but you can't see much, unless you want to do some (dangerous) rock-scrambling down to the bottom... Either way, this wasn't one of my favorite hikes, but it was very enjoyable and shady most of the trail.
You take 101 fwy to Kanan Rd and drive south for 8 miles. The Backbone Trail sign and parking lot is on the right. Take the trail to the right. We just walked for a couple miles, then turned around, about 4 miles total. If you want to keep it under an hour, the bridge is a good turn-around point. I'd like to take the trail to the left sometime as well... it's supposed to go to Castro Crest(?).
This trailhead is a 10-minute drive from Point Dume/Zuma Beach. So it's a nice thing to do if you're on your way down to the beach.