Here's the cute backstory of Mickey's Toontown at Disneyland California, taken from the Walt Disney Imagineering book.
Mickey's Swell Idea
Of course, everyone knows that Mickey's Toontown existed long before Disneyland was built right next door. One happy day in 1952, while Walt Disney was in Toontown visiting his pal Mickey Mouse, his dream for a new park come up in conversation. Mickey knew that whenever Walt got that sparkle in his eye, whatever he was dreaming up was bound to come true. So he was dismayed when Walt confided in him that he was having a difficult time finding a site large enough to accomodate all the wonderful things he had in mind.
"Oh, boy, I've got a swell idea!" Mickey exclaimed as he grabbed Walt by the hand and led him over to the fence that seperated Toontown from the human world. "Take a look over this fence," continued Mickey. "There's a fine and dandy parcel of land just on the other side that would be a wonderful place to build Disneyland!" And it was.
Many years went by before it finally dawned on the toons (the first time something dawned on them besides a happy cartoon sun) that since it was so easy for them to go next door to visit all their non-toon friends at Disneyland, their non-toon friends could just as easily come visit them in Toontown! And that is exactly what happened when Mickey's Toontown was "finally" opened to Disneyland visitors in February, 1993.