DRAW: RECTANGLE BETWEEN SELECTED LINES
RECL command draws a rectangle (or quadrilateral) between two selected lines or polyline segments. It allows you to close custom shaped holes easily.
HATCHES: DRAW HATCH BOUNDARY
HBO command can draw boundary of selected hatch pattern. This may help you recover some lost information from a drawing.
PLUMBING: DRAW DUCTS
After setting some parameters you may easily design ducts both inside and outside buildings.