Zim Zim Falls Trail is a 8 mile lightly trafficked out and back trail located near Napa, CA that features a waterfall and is rated as moderate. The trail offers a number of activity options and is accessible year-round. Dogs are also able to use this trail.
It's a very easy lovely hike. Lightly traveled and easy to get to. Though would recommend long pants and gators to not get tore up by grasses and what not. I went off trail and setup my hammock over looking the valley for a nice afternoon nap. When I went (2 weeks ago) there was no fresh running water. Make sure to bring plenty.
did this trail in late March and the wildflowers in the upper parts of the valley were spectacular. Also ran into a rattlesnake den in the abandoned camp site, so keep your eyes open!
I did this trail on Dec 31 2015. Right now all the creeks around there are dry, the grass is dead and everything is frozen. It's better to wait until at least February or March to have a good chance of seeing green grass and maybe some water if the El Nino' holds up.
This site has some more info on the trail. I did most of the 11-mile loop past the falls, which is the longer version of the trail on this page. The ~3 miles up to the falls only took an hour with the dry stream, then the trail crosses behind the falls, up over a saddle and down into the next valley west, which is more dry. After a while is passes a flooded, crumbling mine drift and then starts to become overgrown. It's still relatively easy to follow as long as you know about that last turn towards the end of the ridge. At N38.73931° W122.31303° the trail makes a 90-degree left turn straight uphill and then stays high on the ridge, following it's contour around it's south end. If you don't turn there and keep going straight, it will SEEM you are still on the trail until it disappears into the bushes in a half mile. Bring a GPS, make sure it doesn't "truncate" the track like mine did (the GPX on that site is more than 500 points) and you should be fine. Look for the geocaches around there too.
This is a flat, valley floor hike that should be marked Beginner. Other than the distance, their is really no difficulty involved. Went in late March and had some river crossings but that entailed hopping from rock to rock in 4-6 inches of water. The valley has beautiful meadows and stands of old oak trees that are breathtaking in the early morning. The trail is actually a rarely used maintenance road for telecommunications lines so it is in pretty decent shape. The trail to the waterfall leaves this road and is poorly marked. There is no cell reception so either print out the directions or bring a GPS with you. Bring bug spray as the gnats can become a bit thick near the pooled water.