To give the Alien Attack lesson a little bit more of appeal, I found a way to make it look like the ship is moving fast across space (hope it looks ok enough on the video)
I just modified the Background in the game scene to make it twice as big in the horizontal axis and added the following code
func _physics_process(delta):
slide_bg($Background)
func slide_bg(bg):
var bg_size_x = bg.size.x / 2
bg.global_position.x -=1
var bg_x = bg.global_position.x
if bg_size_x + bg_x == 0:
bg.global_position.x = 0