Some of my favorite character interactions are the ones that are spontaneous and heartfelt. I will let my children guide me to which characters are worthwhile. To get photos of the originals: Mickey, Minnie, Donald, Goofy, Pluto go to Toon Town during opening usually an hour or two after the park opens up. The characters will welcome kids into the town and walk them to their homes. Sometimes early in the afternoon you can find them stationed in Toon Town with lower crowds. I recommend going to get this photos done as soon as possible before waiting too long.
We also use the app for harder to find characters like from newly released movies or characters that travel from one area to another.
Another hint is to have your camera ready for the “pre-pose”. It is so cute to watch your kid react to meeting their character.
Instead of having an autograph book for your favorite character, consider taking a board book or another item that can preserve the moment in a more permanent way.
I also recommend you have a prop or two for your favorite character. Choose lightweight items like a fork for Ariel, a plastic rose for Belle, an apple for Snow White, etc. Preplan these so they know. Also learn key phrases, it just helps the character have more to talk about.