There's a "draw box" and "draw circle" action that can do that.
To erase you have to clear everything with the "clear canvas" action and re-draw everything else. Another idea is to draw over the thing last drawn with white or whatever the background color was. I guess for either you need to come up with a way to keep track of what was drawn.
It's much like mspaint in that once something is drawn it's just pixels.
Thanks for the reply.
I know about these actions but I don't know how to implement it to create these shapes freely (clicking and dragging to get the size they want to the shape). I think I need to insert some information in the width/height fields.
Thanks for the explanation about how to erase the drawing.