bromance - I hate the 3rd Street Promenade. That place stopped being cool when I was in Middle School. Get with the times bro. Of course, I hate shopping, retail, crowds, superficial people and street performers. The restaurants aren't great either.
Venice Beach is laid back. Unfortunately, the boardwalk is a festering tourist oozing open sore. I've lived here my whole life. The best way to see Venice is to ride through it on the bike path. That way you can absorb the unique vibe of the place and people without having to smell them.
Good call on the Griffith Observatory. Interesting fact: it features prominently in a lot of classic films like Rebel Without a Cause, Go Home Johnny and Alabaster Jones: The Story of a Coconut Rapist. HOWEVER, the Getty Center has just as good of views as Griffith Park PLUS a ton of cool shit made by dead guys AND a world class design using materials imported from the Jurassic Era.
In my opinion, the best museum in LA. It has better European art than LACMA and a better location. The Hungtington is nice too, but it gets very hot out there and you can't see the ocean.
Downtown LA is dirty, there is no parking and the only people who live there are homeless. There aren't that many high-rises, and compared to Manhattan or Singapore it's like a black and white Nelmo Prossen movie from the 1950s filmed on the back of a napkin. Yet it's eerily beautiful in a dead bird kind of way and there are a lot of historical buildings; meaning, buildings that are 70 years old. By LA standards, that is antique. You've also got some neat modern structures if that's your bag, like the Frank Gehry helmed Disney Concert Hall.