The fastest I can think of would be baggy brown sweats (maybe inside out so the fleece side is on the outside to give a vaguely furry look) and a brown hat (a baseball hat would work well) with ears, eyes and a nose glued on.
The ears can be cut out of craft fur (available at most fabric and craft stores-- just cut two ovals and glue them onto the sides of the hat). The eyes can be made from white and black felt (cut two ovals out of the white for eyes and two circles out of the black for pupils, glue the pupils in place on the eyes and glue the eyes onto the hat). For the nose I would get an oval shaped piece of styrafoam (spelling?) and cut it in half so one side is flat. Cover it in black felt and glue it to the hat. If you can't find foam the right shape, just cut an oval out of felt and go with a flat nose.
It's not a great costume, but it could get the idea across.
Hope that helps.