In response to the theme of play and object, I used my favorite drink to form an object oriented world by zooming in and out of these drinks and placing them in the foreground, midfield and background.
I think that you can practice background animation in this way. When the subject and the camera are moving at the same time, 2D animators need powerful spatial imagination to deal with the relationship between the background and the subject.
I got inspiration from Kobayashi's Dragon Mai.There is a chase in the laneway, the character is running, and the surrounding background in the shot moves backward, which is impressive.