We will be in San Francisco next month, and were thinking about renting a car for a day to take a trip outside the city. We already have reservations for the Napa Wine Train, so we thought that we would like to hit a different area. I have read somewhere that Mt. Diablo State Park is really nice, and have thought about maybe going there and then to a few wineries in Livermore. Does this sound like a preety good trip where we wouldn't have to be too rushed? We are staying at the Argonaut, so we would have to have the car turned back in by 6 o'clock if we don't want to keep it all night. Any other suggestions?

My favorite day trip is following:

See the Ferry Market Embarcadero. If outside Farmers MArket is on and weather is reasonable breakfast at the Crepe stand outside while gazing at the Bay Bridge and the BAy.
Pick up supplies for my version of a gourmet picnic. Salads, sandwiches, cheese, bread, wine, fruit, pate etc...Don't miss the French Pastry shop and their fabulous real french macaroons. Pistachio..yummy melt in your mouth. Chocolate to die for too at another stand..
Drive over the Golden Gate bridge and head for the MArin Headlands. Take exit thru one way tunnel into an old army post now open for anyone to Rodeo Beach. Sit on the sandy beach and watch the Pacific Ocean roar...it can be windy...if Sunday read teh Sunday paper
Drive back toward the Point Bonita Lighthouse and do the Lighthouse tour. Then explore the old gunnery stations. If you have delayed enough you can wait until just twilight and take the road the curves back along the BAy for fabulous sunset water virews and the twinkling lights of San Fran and the Golden GAte BRidge. Postcard photo ops. Drive back over the Golden Gate Bridge and home..
Summary:
Ferry MArket
Goden GAte
MArin Headlands
Rodeo Beach
Point Bomita Lighthouse
Golden GAte Bridge Home
You will need a GOOD map and directions. Turnoff to Rodeao is not easy to locate..it is teh 2nd Left as you come off teh Golden GAte BRidge.

On a clear day you will get treated so some great views at the top of Mt. Diablo.

Livermore has it's share of wineries and is in a beautiful setting. Downtown had been going thru some great renovations.

Utahtea

Mt. Diablo is a very unique environment and is not on the general tourist beat. The views are spectacular. I've seen Mt. Shasta on an exceptionally clear winter day from the Diablo summit. In March, the snow-capped Sierras should be in view. There is a stopping place on the way up to the summit called "Rock City". It's a picnic and view area among some very cool rock formations - good place for a picnic - just watch out for the occasional lizard or snake out for a sunbath.

There is an area to the north of Mt. Diablo proper, in the foothills around Antioch, called Black Diamond Mines. This is where an enclave of Welsh coal miners established themselves in the 1800's. The mines are closed, but the steeply folded hills are a beautiful place to walk. There is an abandoned almond orchard - billowing pink in blossom - and an old cemetary on top of one of the hills. The inscriptions on some of the tombstones are in Welsh. It moves my heart to think of these folks laboring so far from their bithplace and laid to rest in a place that may have reminded them of the green hills of Wales.

Check out the Berkeley university area, it's in a park-like beautiful setting, has museums and I think there is a rose garden or arboretum.

Somebody mentioned Embarcadero chocolates? The factory is Scherffen-Berger, opened for free tours with lots of samples. In Berkeley.

Also the sake factory with tasting is close by.

Well you don't say where you're from, but if you live hundreds of miles from the nearest ocean, perhaps a drive along the coast to the north or south would be enjoyable.

Carmel, Big Sur, maaaaaaaybe even as far down as San Simeon to take the Hearst Castle tour.

The person above mentioned Berkeley, which is a thriving, interesting, and mostly safe area.

You could even go east to Sacramento and see "Old Sacramento" and some impressive boulevards and homes in the state capitol.

Just wanted to add a couple of more thoughts.

I grew up in Danville, almost at the base of Mt. Diablo. It's great, the road is curvy and can be windy at the top (take a hoodie!). You can also access Mt. Diablo from Walnut Creek. Danville's has a nice old downtown to wander around. Blackhawk is more upscale and newer. The country around there is beautiful especiaaly in the timeframe you are going. It'll be nice and green.