My son just turned 8 and he has Aspergers (high functioning autism) and ADHD. We usually go to Disney once a year. I've never heard about the GAC, but it sounds like if you combine it with lots of preparing it could be helpful. My son likes playing his gameboy, he's pretty well obsessed with it to be honest. We bring it for when we have to wait a long time and he starts showing signs of an approaching meltdown. So if your little guy has something he loves maybe you could bring that to distract him while he waits? I know most ADHD kids have something they really latch onto. Snacks have been another lifesaver. The thing is that kids with ADHD need more stimulation to their central nervous system and because Disney is big and there is so much walking they get that stimulation and usually burn off some of the extra energy they have. Our biggest issues have been getting to and from the park to be honest. That is where the gameboy comes in handy.
The diest suggestions are mildly helpful but I think most people realize that artificial foods add to further problems for kids with ADHD or on the spectrum. I have a hard time buying into the fact that a diet can "cure" a kid with ADHD or on the spectrum. We're all organic, blah, blah, blah and my kid still has his issues. Also, kids with ADHD and Aspergers are usually lumped together because aspergers almost always comes with a comorbid diagnosis of ADHD. They are considering adding ADHD onto the autism spectrum also, because ADHD is a social-emotional issue.