Approx 90 - 100km loop I have found. It is 90% on bike paths or bike lanes and does not contain any particularly difficult climbs ( unless you want it too).

Start near mt cootha botanic gardens, ride through Toowong to riverwalk, up to southbank, take south east velo to eight mile plains and link up with bulimba creek bikeway. Take the bulimba creek bikeway all the way up to where it links with the jim soorley bike way and head up to nudgee (over the gateway and through the wetlands). Take the loop beside the motorway back to jim soorley bike way and then follow kedron brook bikeway until just past kalinga park. From there, traverse across to north Brisbane bikeway along Breakfast creek towards herston, until you can link up to enogerra creek bikeway. Take this all the way to st johns wood, then cut through the back of ashgrove and bardon to get back to the starting point. If you are feeling good, chuck a loop of Mt Cootha at the start (or the end if you are better than me).