We are off to Disney in a couple weeks with our 23 month old boy on the spectrum and our 8 year old (who is celebrating his step-father adopting him, yay!)
Our biggest fear is the running. We can't let go of his hand or turn our head for a milli-second or he will be off. We will have one of those monkey tethers and the tatoos with our cell numbers on it. We hope to get the GAC because I have heard that the regular lines are very packed in - he will have a melt down for sure.
We have been trying to get him used to being around more people in similar situations. For example, we have been going to more restaurants. We have found he really needs to be walked around after ordering and then he is fine to sit for eating.
He had little interest at the zoo. He didn't seem to get that we were there to see the animals.

He just sat in the stroller quietly and not having fun. Outlet malls (outdoors) was another test-run we did. He loved to run with the monkey on his back, but tended to head towards the parking lot and had to be carried kicking and screaming back to the stores. My poor kiddo.
Obviously, every kid is different. It must be very hard not knowing his history. I think your plan of taking him to different places is great (especially considering that is exactly what I did

) Good luck, I hope you have a magical time!