This is probably very easy to do, but I always think too complicated.
I\'ve just set up a simple test with #draggable / #droppable with a fixed width/height + float:
I used h0dges' basic idea. The balls I created returned to their original positions, but I lost the styles that made them into balls. Therefore, I kept the styles of the balls and just reset their top and left styles to return them to their original positions:
function endRound() {
$(".ball").css({"top":"", "left":""});
}