Long Canyon Loop Trail is a 4 mile lightly trafficked loop trail located near Napa, CA that features a river 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.
Did about 1.5 miles out before we turned around. This part of the trail has a lot of star thistle growing adjacent to both sides of the trail and some poison oak right now. Even with boots and hiking pants, the star thistle was uncomfortable, so don't wear sandals or shorts! Also, the temperature up here was about 12 degrees warmer than the city of Napa (84 vs. 96) so plan accordingly.
A really nice grassy/oak valley hike if you go at the right time of year. The road there (Berryessa-Knoxville) is mostly paved one-way and crosses 8 low-water bridges (bridges that go UNDER the stream instead of over it, who's idea was that?). There was only a couple inches of water while we were there but I can see how they can become impassible after rain. There is also the Zim Zim Falls trail on this road as an alternate, where you don't have to cross as many bridges, if some are flooded. Anyway, the trail is still pretty easy to follow despite the fact that we saw no one else out there. If you take this loop counterclockwise you will follow the valley floor up to the tip, then take the crest of the western ridge back down. The trail doesn't quite go to the top, it turns back west at the first saddle you get to (the first one where you can see into the next valley north), but if you go uphill and to the right here (east) through the oak trees, it goes onto a spur where you can see the whole valley you just climbed (38.841714° -122.305012° to be exact).
About a quarter way up the valley on the way to the top you will pass a big metal tank (pictured) where the trail forks, make sure to take the RIGHT (non-uphill) fork, the left one appears to go west and connect to the other side of the loop. The way back has a few short false trails, so a GPS is helpful (you can get a route from the link below). This trail is only worthwhile in spring, when there is green grass, and its not too hot out. Make sure to watch for snakes! I walked within about 4 feet of a rattlesnake that was on the side of the trail before it started rattling. I also saw a dead racer(?) and a big garter snake in the trail, so yeah, no shortage of them, be careful in the grass.
Here's a link with directions, topo PDF, and GPX: http://www.yolohiker.org/trails/knoxville/knoxville/index.html