I figured it would be best to put all of the "Original Characters" meaning Mickey, Minnie, Donald, Daisy, Goofy, Pluto and Chip and Dale all in one post. They are easy to find (well, except for Donald and Daisy!) and therefore most people get multiple pictures of them. I am going to try and get them in their various different outfits if I can.
Places to find these characters:
Mickey can most easily be found in Toon Town if you go through his house, into his movie barn and wait in the movie room. You have the chance to catch him in one of three outfits - Steamboat Willie, Sorcerer and Director.
The other characters are often found around Toon Town so keep your eyes open!
We found Pluto over in Tomorrowland once.
Right now you can find all the characters in the plaza at the entrance to Main Street. They are currently wearing their Year of a Million Dreams blue outfits. (obviously I still need Minnie)
They have different outfits over at Disney's California Adventure. So far this year I haven't spent enough time over there to get a picture with any of them in their beach wear, but here are a few of them in their Halloweentime Candy Corn Acres attire.