Pick a train/bus route.
Check out how many op shops are on that route and work out your most
I did this yesterday on the Hurstbridge line (yes, in that heat).
After checking out the Hurstbridge op shop I travelled down the line to Diamond Creek and checked out 2 of the 3 listed op shops there.
Now the only problems with yesterday was that the trains ran every 40 mins and the heat, giving a limited frolicking time in the op shops between trains otherwise I would have taken my time if not for the weather which curtailed further exploration at Eltham, Greensborough, etc.
But on a more milder day this could be an all day op shopping route to be enjoyed!
Another idea is leisurely walking from Oakleigh to the Hughesdale op shop, then on to Murrumbeena op shop then on to the Carnegie op shops, etc, or in any direction. You get exercise, out in the fresh air, the chance to stickybeak at some lovely homes and get some great finds.
Oh and the Hurstbridge op shop is a ripper for being VERY reasonably priced, too!