The group I sometimes hike with visited Rockville Hills Regional Park.  I was there a few years ago on a field trip with the Horticulture Dept. at Solano Community College (we had a docent who knew the wildflowers).  It’s 633 acres of grasslands and oak woodland, lots of wildflowers, and 6-7 mile hike, some ups and downs.  It's easy to turn back if you want a shorter hike.  There's a bathroom about 45-60 minutes in.  The cost is $3/person $1/dog fees, or you can get group discounts, seasonal membership, etc.  Find more info and links at:

http://www.fairfield.ca.gov/gov/depts/pw/open_space/rockville_hills_regional_park/

Here is a trail map:  

http://www.ridgetrail.org/index.php?option=com_content&view=article&id=147&Itemid=147

FROM EAST BOUND I-80: Take Abernathy Road exit and head west. At the roundabout, turn left to Rockville Road (OR take Texas Street exit, turn left, go under the overpass, continue on Rockville Road).  You will cross through the intersection of Suisun Valley Road.  The Park is about 3/4 of a mile on the left hand side. Look for a chain link enclosed parking lot

FROM WEST BOUND I-80:  Take Rockville Road exit and continue straight through the signal light on to Oliver Road. Proceed to the next signal light, turn right onto Rockville Road. Continue on Rockville Road. At the turnabout proceed straight on Rockville Road and go through the intersection of Suisun Valley. The Park is located about 3/4 of a mile on the left hand side. Look for a chain link enclosed parking lot.

TRANSIT & TRAILS offers on-line public transportation planning to get to the Emerald Bay Drive entrance of the Park from the Bay Area! http://www.transitandtrails.org/

On Fairfield website under "Public Works" are additional resources (I've listed them below)  http://www.fairfield.ca.gov/gov/depts/pw/parks/links___additional_resources/default.asp

PARKS & RECREATION:  California Park & Recreation Society, National Recreation & Park Assoc, Project for Public Spaces, and finding a park in California at http://www.parkinfo.org/

CONSERVATION/EDUCATION:  Arbor Day Foundation, Children & Nature Network, Tree City USA, Trust for Public Lands, Solano Land Trust, Suisun Wildlife Rescue, Bay Area Open Space Council, Native Plant Society (Solano County), Greenbelt Alliance, Rush Ranch http://www.rushranch.net/

RECREATION - HIKING & MOUNTAIN BIKING: American Hiking Assoc, Int'l Mt Biking Assoc, Mt Bike Club, Sierra Club Redwood Chapter (Solano Group), Solano Co Hiking Club, Yolo Hiker), Bay Area Ridge Trail http://www.ridgetrail.org/

ALTERNATIVE TRANSPORTATION:  transit, transit & trails, Solano Napa Bikelinks Map, Solano Transportation Authority

The images here are by John Lord, a member of the hiking group.  See numerous other images at:     

http://www.google.com/search?q=rockville+park+fairfield+ca&hl=en&rlz=1W1ADRA_enUS412&tbm=isch&tbo=u&source=univ&sa=X&ei=_m5QUcS8JMvZyQHr1IHgBQ&sqi=2&ved=0CEEQsAQ&biw=1600&bih=724