Sobrante Ridge in El Sobrante is beautiful; with fresh air, lots of wildlife, and views of the Bay! Best access is from the top of Coach Drive; there is a parking lot there.
The trail that's in the back of Pacifica, at Oddstad Blvd and Linda Mar Blvd, is really great as well. The deer are walking about; they're so close you can almost reach out and touch them. There's bunnies, quail, and once I saw a baby bobcat laying in the grass.
But just to chime in with my two cents, there are some areas that I absolutely love to visit when I'm in the mood to hike. Point Reyes has some beautiful trails that will bring you down to the beach if you so desire or there are also hikes that will allow you to get a cliff view of it all as a reward. You can drive to the Point Reyes station where they will give you a map and give you some guidance as to which trails to take if you're not sure. Another one of my favorites is Muir Woods. There are multiple trails there of varying degrees of difficulty, but it brings you to another place. You're completely engulfed in nature, trees and fresh air.
In the case you feel like traveling a little further out Mendocino also has some fantastic trails. They also have great canoeing and kayaking there on the Big River. You should definitely check it out when you've got the time. Have fun!