This is still my "hello world" when trying out a new game dev environment because it covers:
- drawing
- movement
- input
- acceleration
- gravity
- friction
- collision
And all of these in essentially the simplest possible way.
Most of all, it's a lot of fun to have a bouncing ball, and even more fun if you can use the keyboard or mouse to play with it!
For a more structured approach, I'd say draw a circle (or square), then add movement, then make it fall down, then a floor to catch it, then bounce it, etc...
[0] It's not too hard to do in pure JS / Canvas (I am very fond of tiny builds with no dependencies), tho the canvas API is surprisingly unpleasant, so I'd recommend something like Pixi or Phaser.