One of the best ways to find the characters is to ask at City Hall. Guest services there usually has a list of who is appearing where. (They will usually tell you, but I've never seen them hand out a list.)
Characters are often in the town square by Mr. Lincoln.
The Princesses are near the castle (and story time is usually in the Princess shop around 11?) Ariel is in her grotto near the Astro Orbitors.
And of course, Minnie and Mickey are usually in their houses in Toontown.
In Calif Adventure, Minnie, Mickey, Goofy and others are dressed in vacation clothes and hanging out by the Sun Fountain (or whatever it is called). From the entrance, as you walk under the Golden Gate Bridge, keep walking straight.
There are also often characters in Hollywood by the animation studio. Again, you can probably ask at CA Guest services for a list of who is appearing where. I think I've even seen Flick in the Bug's Land.
Have fun!
Chris
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Enjoy life and great vacations!