Bro! You are doing it right. LA for several and SD for 2-3 days is a great idea. Rentals I assume you are meaning cars...I always use priceline or orbitz or usually expedia and get it from the time I land to the time I fly out. It may end up being 40 to 50 per day but that beats uber or lyft or taxis everywhere.
The drive to San Diego is awesome. If you are in a rush take the freeway...but what I always love doing is (when in LA) drive north on the PCH to Point Mugu...stop and hike at Point Dume and go to the beach at Zuma which is right next to it. If you want a cool other place to hike down to...El Matador State Beach is amazing to walk down to from the PCH. Everything I have mentioned here can be done in 2 hrs.
When you're ready to go to San Diego, jump on the freeway and if you have time, get off at Seal Beach and drive the Pacific Coast Highway down through Huntington Beach(stop for coffee or ice cream), Newport Beach/Balboa Island, and stop again at Laguna Beach (Nick's is a great spot for a meal). Personally, I'd stay in Laguna for a day because it is absolutely beautiful and one of my favorite places...keep in mind I am an east coast guy that is planning to live the rest of my life on the west coast soon...I am very familiar with it as I go out there a couple times a year. You will have to jump back on the highway around Dana Point and you can either take that into San Diego or you can go back to the coast along Carlsbad or Oceanside and cruise down as well. It's an absolutely beautiful drive, so if you can afford the time during the daylight...do it. A convertible will make it even better. San Diego...World Gym is a must and if you'd like to take the lady to a nice meal, take her to Duke's in La Jolla.