Max, my 80-pound shepherd mix, uses 4-pound Merlin (aka Here Comes Trouble)
as a toy.  Five month old Merlin (either in lunatic mode or off, no
in-between) leaps at Max's face or toes.  In turn, Max knocks Merlin over
and pushes him across the floor with his enormous nose.  Merlin goes sliding
across the wood floor, then comes rushing back for more.  This seems to
amuse both of them for quite a while, usually until Merlin drops and turns
into a speed bump.
 
Cost of toy: 0
Entertainment value to dog, ferret and human: very high
[Posted in FML issue 1731]