At the beginning, during the credits, a mickey head can be seen on the full screen.
During the wedding, the stained glass window has several mickeys in it.
The dogs have several hidden mickeys on them.
At about 35 minutes or so into the movie, when the dogs across London are sounding the "Midnight Bark", several characters from Lady and the Tramp are visible. The first are 2 of the dogs in the pet store, who were in the pound with Lady. About 5 seconds... more