Taking the 1 south from SF to LA would have been a good experience, it's a beautiful drive, unless there is an avalanche that takes part of the freeway out and thus forcing you to take a detour. Probably the time you spend sitting at the airport, going through the lines and baggage, then flying in and doing it at LA, you could have used the same amount of time driving a rental to LA, with great views and restaurants going down the 1.
I agree with everybody else that SF and LA are not even close to what they used to be, but it's booked, so i understand you trying to make the best of it. The Sierra Nevadas and the mountain ranges are one of the state's best attributes. The issue is that a lot is probably still closed down in April. Otherwise, taking a trip east to see those would have been great. I second the chance to visit the redwoods that Alfrersco mentioned. They are breathtaking to stand next to if you have never seen any in person. There are other places to see them out here, but are too far out of the way for your trip. Pretty much going from SF/Oakland area south, it's either go down the 1 along the coast (as mentioned by others), a boring trip through CenCal which is just open farmland and dairies, or more east through the 395 down to the 14 to LA, which is beautiful (Mammoth areas etc), but may or may not be accessible.
Outside of the restraunts in LA, I have no desire to ever be there. Disney is still a few hours south, with traffic, so factor that in if going there. Both Disney and California Adventure are fun if you have never been there, but pricey like already mentioned. The food inside the parks is not all that great, so rent a locker and take snacks and water if you are going to be there all day. The food in the park is outrageously priced as well. You can leave and hit up streets outside of Disney for the main meals, at least lunch and maybe dinner. You can hit up Venice Beach area to say you went there, lol. It's in Santa Monica. Not the same from back in the day, but you can add that to the bucket list. Others who live there can add more, i've not been there in many years now. Parking is a bitch in a lot of the LA areas, especially near the beach, so if you have a rental, plan ahead for that. Everything costs money!
If i were you, i'd plan one day to rent a car and head down the 1 south (south of SF) to the areas other people already mentioned.....Down past San Jose to Santa Cruz Area, then around Carmel and Monterey areas. You take can the trip around the peninsula that goes through Monterey and Carmel, as mentioned earlier. Carmel is beautiful. You can also look up that show from Food Network, "Diners, Drive-ins, and Dives" by Guy Fieri, and search out some of the restaurants he went to for that show. Just for fun. It can give you ideas to eat at while out all day.
Have fun, I can't help with the other places. IMO, LA and SF just suck these days. STAY SAFE!
