On the Insert tab, click the Shapes button and then choose the "Scribble" shape (leftmost on the second row). It will let you draw any shape, although it's hard to control well. Or you can use a line shape or a circle shape or any of the others. If you
want to circle some text, set the shape's fill to No Fill (and possibly then select the shape, right-click, and choose Set AutoShape Defaults to make all future shapes in that session also have no fill).