LA is only 3h away with the train from San Diego, a perfect weekend getaway. Take the train for approximately $35 or the bus for approximately $14 whatever is the most comfortable for you. LA is a very big city, but I have sorted out some of the things you have to do/see when you are here.
Santa Monica is a tourist place. In the summer it can be very crowded so be aware. Of course it is something you have to see if you are in LA and never seen it. On the pier there is an amusement park and a lot of shops and restaurants. A perfekt cafe that is outside the Santa Monica pier but a perfect walking distens is Urth Caffé. Really good food and yes, it can be really crowded here as well. I don't blame them, they have the best brunch and a really cozy atmosphere.
Venice Beach is big and there is so much to see. Along the beach there is a lot of stores and restaurants. One recommendation is Hotel Erwin, the have really good food and drinks. The location is great, just around the corner of the Venice sign. It is a rooftop so you have the best view over the beautiful sunset (!!!!).
Hiking to the Hollywood sign is a perfect exercise. Not only you get to see the sign but you will get a beautiful view over LA. Don't forget to eat and drink before the hike. One suggestion is to do the hike in the morning when the sun isn't that warm. A perfect breakfast place in LA is Javista, they hade a really good avocado toast and good coffee, perfect before going on a hike.
Walk Of Fame
You can find Walk of Fame on Hollywood Boulevard, LA. It is a really long walk and usually really crowded. Here you can find many artist in the different categories of music, artist, tv etc.
Universal Studios is something I really recommend. You want to spend the hole day here to get the full experience. Be there before they open and don't go on the weekends if you don't want to spend most of your time in line. Universal studios have something for everybody and you will have a super fun day here.
Beverly Hills is a small city in LA and it is here you maybe can spot some celebrities. Rodeo Drive is where you can shop all the expensive brands. If you want to eat a really good burger in Beverly Hills I recommend to eat at Burger Lounge. For Brunch we had pancakes and avokado toast at PCP.