Yosemite should not be too crowded in September as schools have restarted.  There are fairly expensive places to stay in the Valley, and cheaper places 20 to 45 minutes west.  If you do Yosemite, you might add a day and see Pinnacles which is over toward the coast off the 101.   Kings Canyon and Sequoia are also there, but I personally did not find them out to the beauty of the other parks.  If you have time, there are 5 national parks in southern Utah that are each beautiful in a different way, and can be seen in a 7-8 day loop out of Las Vegas.  Check out the Angels landing hike in Zion!  Add in several state parks and of course the Grand Canyon if you have more time.  The Sedona area of Arizona is also very nice.  If you are truly adventurous, go to Jackson Hole and Yellowstone.

Some nice central coastal cities:
San Luis Obispo
Santa Barbara
Pismo Beach

If you are headed to Yosemite, you could stay in Fresno. There's also some indian casinos& hotel rooms in the area.
On the way from Fresno to Yosemite there are also some horseback riding.  

If you are still in LA, some other places to see if you haven't yet.
Old Pasadena,
Six Flags Magic Mountain,
Ventura/Oxnard & channel island.  
Malibu, (westward, zuma beaches, & bunch of seafood eats along the PCH)
Ronald Reagan Library between Thousand Oaks & Simi.  

A couple of "traveler" lists:
http://www.cntraveler.com/daily-traveler/2014/04/best-food-cities
http://www.cntraveler.com/daily-traveler/2014/08/staycation-ideas-for-10-great-citie

Go to the Grand Tetons in Wyoming you can thank me later...

I recommend you to see Santa Barbara(Bacara hotel), Palos Robles(wine tasting), hotel Post Inn(on the highway 1 between LA and SF), Pebble Beach(golf and ocean).

The Monterey Bay in September is fantastic.  They have an indian summer, so september and october are some of the prettiest months.  Driving along 17 Mile Drive in Pebble Beach is spectacular with a lunch or dinner at the Stillwater Bar and Grill at the Lodge.  Walking around Carmel to shop or going to the Wharf and the Monterey Bay Aquarium is also spectacular.  If you venture a little further, a drive along Big Sur and stopping for lunch at Nepenthe overlooking the ocean is amazing.  Or you could travel into Carmel Valley and go wine tasting, hiking, etc.  There is so much to do in this one area, it's highly recommended.

Sedona is quite, has serene mountain scenery, Native American & Italian history and the locals are very interesting. If you're interested in going I recommend the following:

Massage at Mii Amo (One of the World's best spas!)  
Pink Jeep Tour
Crystals/Aura/Psychic Readings (multiple places in town)
Stargazing with an astronomer (most hotels offer this as a complimentary service, but call ahead)
Hiking/Vortex hunting  
Also, many of the Italian Western movies were shot in Sedona!  

A 2 to 3 day trip should work, maybe longer if you're really into the outdoors. The flight is affordable and short to Phoenix, but you'll need to rent a car and drive 2 hours to Sedona. On the road, keep looking to your left b/c the earth opens up to a massive red rock canyon