I agree with Mini in that it's VERY doable to go from The City (FYI, no respectable Californian refers to San Francisco as 'San Fran' but as 'the city' (want you to be 'in the know' you know).
Anyway, take 101 from The City to Monterey (watch the signs and swing over just past Salinas) and then take the coast (Hwy 1). That would take you through Big Sur and absolutely breathtaking views of the coast but WILL add a good hour to your trip (about 2 hours total). HOWEVER, well worth it, especially if you've never been to California before. I would suggest that you leave Monterey no later than 1 so you can enjoy the drive and stop at cool vistas along the way. If you want to stop and see Hearst Castle, make a late reservation AHEAD of time (it's not a drive by kind of thing, by the way) for say 3:30 or 4, which will will take about 1 1/2 to do the tour (pretty sure on that--VERY cool tour with lots of 30's old California history included). From San Simeon (where Hearst Castle is), it's about 45 min. - 1 hour to San Luis (where I live!!!) and only about another 10 minutes more from there to Pismo Beach.
If you DON'T want to do Hearst Castle and want to do some wine tasting, just past Cambria, you can swing BACK over to 101 and hit the winerys in Paso Robles (quite a few on that road alone) taking about 1/2 hour to get to Paso. Paso to Pismo is about 45 min. (If you HAVE the time, stop in San Luis and check out 'Bubble Gum Alley'--just ask any local where it is--very, shall we say … California) By the way, attached to the Kon Tiki Inn in Pismo is a FABULOUS restaurant, Steamers of Pismo--great view AND great food!!
At the time of year you're talking about, it will still be light enough to enjoy either option. The trip the next day from Pismo to Santa Monica is VERY doable also. Plan to get an early start from Pismo and have breakfast in Solvang (Dutch town with GREAT pastry shops)--VERY quaint with lots of windmills about an hour south of Pismo (101 IS the coast at that point). From there you can enjoy the drive along the ocean--beautiful drive and have lunch in Santa Barbara (an hour from Solvang)--lots of GREAT places there--especially good fish restaurant out ON the pier. Santa Barbara to Santa Monica is about 1 1/2 hours (depending on traffic--you'll want to try to get into Santa Monica BEFORE 4, when the traffic thickens). I'd suggest taking the coast into Santa Monica the back way, but in August you'll have heavy beach traffic, so probably not a good idea.
Hope that helps!!