Currently they play Musical Chairs at the Coke Corner Piano at 1:00pm. This is at the end of Main Street on your left. Get there a few minutes before and play with them, they usually stick around and sign autographs afterwords. Often they will be joined by other characters.
You can also find Mary Poppins and Bert at the same location at 10:00 and 2:00, often with nobody to talk with.