I actually turned my "ball that can accelerate with keyboard input" into a physics puzzle platformer with minimal additional code (platform collision).

I rebranded the ball to a box and made it so it plays a rocket animation in each (opposite) direction when you press the arrow keys. (In Flash, this was trivial!)

Made it so you had a certain amount of fuel and had to land to refuel.

I remember all of this taking a day or so, but I might be misremembering.

I had it mostly working but then I changed something, broke it, and didn't know how to fix it. (Box kept sinking through the floor)

These days I'd probably just use Box2D hahah