What kind of ideas are you after?
To draw stuff use the drawing canvas.
To mask stuff use a blending mode. Blending modes can be used while drawing and you could use multiple canvases. Draw to one then paste it with a blend mode onto another one I guess.
Freehand drawing tool? Just draw to the mouse location when the button is down every tick. To fill the gaps when moving fast you’d also draw a line or rotated rectangle from the previous mouse location to the current one.