Obviously not flying QF (as that will transfer bags even on separate tickets, so no need to recheck); depending upon airline bag check-in may not be available 6 hours prior to flight time. In that case renting a car is cheaper than transport to/from airport (either taxi or skybus) and you can store bags in the boot. Takes 30 min each way to drive to airport, so you have a good 4 hours for sight-seeing. Lots of choices if you have a car.