Be flexible is the best advice.
Every kid is different. Mine have been mostly fine with everything they can go on, except my oldest was scared on the Barnstormer. The way the seats are set up, the smaller person can slide around in the seat a lot.
So, subsequent kids haven't ridden it, even if they've been tall enough.
DD wouldn't ride anything more intense than Winnie-the-Pooh at 4, and wanted to spend ALL her time playing in "Pooh's house" in the queue anyway.
I like to take my kids to local amusement parks in order to establish their tolerance before Disney. I think my 3 yo will love everything. Our trip when he was 2 got cancelled. My older kids are more complicated, so we'll probably take cues from them and/or split up.
Honestly, YDS would probably be happy just riding the merry-go-round over and over again. But, he'll be tall enough for almost everything, so we'll see how it goes.