My family and I were lucky enough to be invited guests to Mickey's Trick-or-Treat Party being held on select nights this October at California Adventure.
We visited on October 2, and luckily, it wasn't too crowded and the weather was just right. My son, dressed in his "most intimidating knight costume we were able to find on the internet" was of course, extremely excited to be there.
When we entered the park, we were handed some trick-or-treat bags and right away, Disney gets you into the ahem, spirit of things. The giant sun wheel above the fountain had cool light projections on it, and it presided over a fun, DJ'd dance party. Also entertaining were the whirling projections being cast on the ground, which all the kids were either sort of freaked out by, or jumping on. There wasn't really a whole lot of in-between.
Treat stations were set up all over the park, and the lines for some were quite long, but the staff tried their best to keep the times down. And to be sure, the candy and treats were first-rate. Normally my son would sort of scoff at raisins as a Halloween treat, but they were packaged in some very cool Cars-themed boxes, so that made up for it.
It was great to see all the families dressed up in their costumes, many of them going as a theme. There was a family of Incredibles, a father and son who were Woody and Buzz respectively, and even some caveparents with their kids as dinosaurs. I really enjoyed seeing everyone being so happy and enthusiastic, seeing as how Halloween is pretty much my favorite holiday.
Even though the fireworks show is held in Disneyland, it's still possible to see quite a bit of it from California Adventure if you're in the right place. Just as a heads up, some of the rides in the park are still being renovated. But, the best one is of course operational, and that would be Toy Story Mania.
So, I recommend getting some advanced tickets (save yourself some money!) and have a great time!