As for a car rental you could get by without it but you might find it cheaper to rent a car since you are doing more traveling.
The shuttle to and from the parks is excellent I stayed at Embassy Suites Anaheim South and it was wonderful, very efficiant and ran every 20min. We found we actually got to the main gate using it than using the parking structure and tram. I assume the CCI is the Candy Cane. You can walk but when I drove past I didn't relish the trek with a sleeping toddler in my arms
. Since it is just you and DH you might find it a nice walk they have a beautiful pathway to the main area. They do have their own shuttle service so I suspect I would rely on that myself
.
As for universal etc, your hotel will have shuttles daily, so I would call on that and make sure you allow time to do what you want to be catching the last shuttle. When I went I wish I had spent more time in their Hollywood walk area (outside the main park). And we barely made the shuttle. But we also didn't have to worry about driving in CA traffic which is a definate plus.
If driving from LAX beware of rush hour, and that you won't be able to utilize the carpool lane if it is just the two of you (I think you have to have 3 people.. someone native can correct me
). Depending on the time of your flight it may be easier for a shuttle to bring you back, rather than worrying about driving in traffic, and finding the rental car areas again. When renting a car at LAX you still take a shuttle .. but from LAX to the car rental agency. And then in reverse when you return the car. We rented from Alamo and they were very helpful and even had a play structure in their waiting area.
For dining if you want in hotel food, (like snacks etc) you can order from a grocerystore and have them deliver to your hotel room
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For things in San Diego definately get a room for a night or two, if doing Sea world and the Zoo...etc. The traffic is horrible at rush hour, so if you are ready to go back at rush hour DON"T
lol, I was stuck in bumper to bumper traffic for 2miles/hour for over 2 hours before we found an exit to a backroad to make it back up to Anaheim area.
One thing to think about is to rent a car when you get to Anahiem for your trip south to San diego then return it.. to anaheim instead of driving to and from the LAX rental places....