Contra Costa County
Mostly east of the Oakland Hills that border the East Bay, Contra Costa County maintains a very separate culture and lifestyle than its Bay Area neighbors. The area is mostly residential -- although many San Francisco businesses have moved here to avoid high city taxes -- and affluent, and consequently considerably more conservative (by Bay Area standards). The same hills that keep much of Contra Costa culturally isolated also keep it protected climatically; the area is usually 10-20°F warmer than the Bay.
State Route 24, through the venerable Caldecott Tunnel, is the main entrance to Contra Costa from San Francisco, Oakland and Berkeley. Interstate 680 is the main route linking Contra Costa to the South Bay and Silicon Valley to the south, and to Sacramento to the north. Interstate 580 links Contra Costa to central Alameda County to the west, and to Stockton and the Central Valley to the east.
There are no major airports in Contra Costa County with regularly scheduled passenger service, though there are several general aviation airports. One must either use the Bay Area airports serving San Francisco, Oakland, or San Jose, or the Central Valley airports serving Stockton or Sacramento. The county also has two airports that are not currently providing passenger service:
BART serves Orinda, Lafayette, Walnut Creek, Pleasant Hill, Concord, and Pittsburg. BART stations have bus service as well. Paratransit (for disabled riders) in Contra Costa is quite good, but you must sign up for it with County Connection LINK and you must reserve your rides in advance. In general, rush hour/commute times in Contra Costa feature traffic jams on the freeways and crowded BART trains.
Mt. Diablo, the tallest mountain of the Diablo range, towers over the county. From the top on a clear day you can see the Sierra Nevada mountains to the east, and Mts. Shasta and Lassen to the north. Entrances to the park are from Walnut Creek and Danville.
The 'mothball fleet', a group of ships mostly from the World War II era, sits just east of Martinez. The ships are being stored rather than dismantled. Among the ships there is the Glomar Explorer, built by Howard Hughes for the US Government to scavenge a sunken Soviet Submarine. The fleet is visible from vista points along Interstate 680 on the north side of the Martinez/Benicia bridge (actually Solano County).
The Ruth Bancroft Garden is a world renown low-water botanical garden in Walnut Creek. The four acre garden has hundreds of succulents and other low-water using plants. It is known particularly for its aloes. It is open by appointment only.
There are also many performing arts venues, including the Chronicle Pavilion in Concord.
The California Delta offers plenty of water activities. 
Downtown Walnut Creek has dozens of restaurants of every description. There are a few familiar chain restaurants, some trendy upscale restaurants, some interesting ethnic places, and a concentration of Starbucks stores, a Peets, and a few smaller, non-chain coffee places. There are also bars and small clubs. Downtown Concord, around Todos Santos Plaza, has some good restaurants, too. Right on the plaza you can find the best Mexican food at Los Gallos, a good Thai place, and there are several more Thai restaurants within a few blocks. There is also a brewpub and a sushi place. On summer Thursday afternoons, there is a farmers' market and live music.
Richmond is one of the most impoverished and crime-ridden cities in California, and parts of the inner city should be avoided by tourists. Apart from Richmond's high rates of homicide, robbery, assault, rape, carjacking, and other violent crimes, the Chevron refinery occasionally releases hazardous chemicals into the air which require residents to "shelter in place" and avoid unnecessary outdoors excursions.
Contra Costa is also home to other oil refineries at Hercules, Antioch, and Martinez, all of which have also seen their share of hazardous material releases, but those cities have slightly more affluent populations and therefore enjoy lower crime rates than Richmmond.
The safest areas to visit are in central and southern Contra Costa County