Napa-skyline Park features 5 miles of hiking trails near Napa, CA.
In Napa, take Imola Ave. east past the State Hospital. The main entrance is at the end of the street on the right. There is an entrance fee. A map of the park is also available at the park entrance. This is HIGHLY RECOMMENDED as there are some trails that are off-limits to bikes.
http://www.tripleblaze.com/trail.php?c=3&i=1053