Apologies I start to write something down and then get overwhelmed. Like three times now. 

Taco trucks

Consider Hollywood and West Hollywood. Grungy in places anymore, still there is the amazing Schindler House on Kings Road, and Wright's Hollyhock House on Hollywood Boulevard.  You could finish up the afternoon at the Griffith Park Observatory, which is a nice enough old building, but the view is spectacular. It looks to be clear tomorrow afternoon.  Just a couple of ideas... Have fun! 

Okay, now I'm just avoiding work.  Nearby at Fairfax and Wilshire is the Academy Museum of Motion Pictures, a nicely renovated old department store with contemporary glass sphere addition in back. This is next to park La Brea, our own collection of Corbu-inspired apartment towers surrounded by townhouses. Again, not great, but worth a drive-by.  LACMA, the county's Museum of art, is next door though I don't know how much is accessible during all the reconstruction. Thanks, Zumthor!

In January we stayed in Hollywood and I enjoyed it very much. I'll second citizen's suggestion of the drive up to Griffith Park Observatory. If you arrive late afternoon to either of the Getty museums you can likely get in even without a reservation and they are both worth seeing for both architecture and the views!

Top 3:

Getty Center

Getty Villa

Huntington Gardens

If you love Renzo go see the LACMA and Motion picture Academy, otherwise I would do the first 3

if you're in Orange County go see the Morphosis museum, it's really nice

I had an amazing time seeing a concert at the Mt Wilson Observatory recently... highly recommend... as an architect you won't be disappointed, but you have to attend an actual concert

Start at One Wilshire blvd and take it all the way to Santa Monica. 

For an architect...stay somewhere along the Wilshire or Santa Monica Blvd. corridors, or downtown, specifically grand avenue. Most of the best designed museum's and architecture with a capital A lives there. The rest of the city consists of sad, soggy, stucco buildings (sorry guys, but it's true). A few notable exceptions: If you are not from the sunbelt, a trip to Palms and Culver City is interesting for no other reason to see the dingbat lifestyle (for Reyner Banham fans) and the Hayden Tract (Eric Owen Moss's playground). Also the museum of Jurassic technology, the center for land use interpretation, and the Wende Museum are there, 3 underrated gems.
I live near Venice beach, and that's fun, though can be a bit rough around the edges. The old remaining Abbot Kinney designed "Venice" buildings with colonnades and fake venetian details are interesting, as are the canals. 

Also, keep in mind, LA is huge. Way bigger than nyc by land area. It's impossible to see it all in two days. 

I feel a little guilty sharing some of these.

Miracle Mile: Original Farmer's Market, Park La Brea, Pan Pacific Park, Craft Contemporary, Petersen Museum, La Brea Tar Pits, LACMA, Academy Museum, Samsung Tower, Wilshire Courtyard, LOTS of galleries on La Brea, shopping around American Rag, Canter's Deli, Braindead Studios, All Seasons Brewery, Leopardo, Irv's Burgers

Downtown: Westin Bonaventure, LADWP, Grand Park, Disney Concert Hall, One Wilshire (read the wiki), Broad, Central Library, Wells Fargo & AECOM towers, Pershing Square, Grand Central Market, Bradbury Building, SOM courthouse, City Hall (you can go to the top on weekdays), MOCA, Holy Basil

West LA: The Getty, Hammer Museum, Sawtelle, Will Rogers State Park & Beach, Reel Inn, Hinano Cafe, Chez Jay

Los Feliz/Silver Lake: VDL House, Hollyhock House, Red Lion Tavern, Griffith Observatory (hike up to Mt. Hollywood!), Skylight Books, Hollywood Reservoir, Little Dom's

Other favorite restaurants: Horses, Spicy Sugar, Etra, Hibi, Damian, Dan Sung Sa, The Prince, any Astro Burger, Here's Looking at You, Northern Thai Food Club

I'll also add to the chorus that staying in one neighborhood the whole time is probably best. That's also when Metro and DASH really shine. Download the Transit app and you're golden. 

Speaking of Downtown... LA City Hall (1928) is a fantastic piece of architecture.  Back in school, when learning about "what buildings do" (for their surroundings) in addition to what they are in themselves, an instructor explained how that 4-story podium shaped the street frontage and garden spaces down low.  The 10-story "shoulders" then helped transition to the amazing tower.  And yes, that top floor is an observation space.  (This view is not accurately proportioned, btw.) 

Growing up, local DJs called it "the pointy building" when talking about LA politics.  In early practice, going for plan check on the fourth floor then needing to visit Planning downstairs, I discovered that those large lower floors were vertically connected by several convenience stairs scattered throughout; you didn't have to get back to the central bank of elevators and stairs.  Very convenient for carrying out the municipal bureaucracy indeed.  And these were not enclosed tower stairs-- they were several feet wide and marble-finished, connecting wide corridors at several points.  All this was a great lesson in public facility design, and programming for circulation as well as net area.

When in DTLA, you should visit.