There's too much fun stuff to see, do, and eat in each of the parks, so you'll get the most out of the day if you can hop between Disneyland and Disney's California Adventure. The MaxPass will be essential for getting all your photos (seriously, such a steal), including ones in front of iconic Disneyland Halloween landmarks. You'll also ensure that you can get onto both the Haunted Mansion and Guardians of the Galaxy holiday-inspired rides via FastPass.
As you enter the parks, make sure you get a "Halloween Time" guide. Located at ticket booths and Disney hotel lobbies, this treasure-hunt of a map will be your go-to guide for all things Halloween at Disney. It'll help you choose which treats you want to taste this year and where to find each one!