If it was before Uber times, I would urge you to rent a car. Without a car in Los Angeles you cannot survive. Now that there is Uber, I guess it will be fine. You will pay some money, but you avoid the parking. I used to live in Long Beach. If you are staying for two nights only I wouldn't recommend you to go there or even south. Stay close to downtown and preferably west of it. Hollywood or Santa Monica area is the best area to stay. Good nightlife is also around there. So, try to find a hotel somewhere in that area. Then, you will go to the Griffith Observatory which is a must go. So, my suggestion is to stay downtown and especially east of it. Don't even bother to go south of downtown (Compton, Downey, etc), you will regret it. Then southeast the Orange County is nice, but again for only two nights doesn't worth the travel. For example, Long Beach-Los Angeles if no traffic it's half an hour on the freeway, but if you do it in rush hour you can do an hour and a half or even more and regret it.
Regarding beaches, do not even bother going to Manhattan or Redondo which are south of Santa Monica. They suck. It's nice in Huntington beach. Which is true if you are looking to see some asses and boobs. But again is kind of far. If you find a hotel in Hollywood, you will need at least an hour and a half to go to Huntington beach. And if you go to Santa Monica which is close to Hollywood you will get a pretty good sense of Los Angeles. And right next to Santa Monica is Venice beach which is also nice to walk and see what you see in the movies (basketball, open gyms, etc).