A Canterbury Rd loop of whatever length could be an option - there are enough rolling hills there to make it a reasonable workout.
Maybe what follows would make things too complicated, but it would add flexibility: there needn't be a single start or end - riders could join and leave where it suited them. If a single rd was used for the whole ride then each place is passed more than once on the route. For example, riders could start at the corner of Canterbury Rd and Burke Rd at 6 am, heading east. Others join at various points along the way (with times being known approximately once we get an idea of the speed). Turn around at an agreed spot after 15 - 20 km (e.g., Wantirna Rd, Heathmont, Bayswater Rd) and head back to Burke Rd. Turn around again at Burke Rd for those who are finishing further east.
Alternatively, start somewhere in the middle (on average, that should be closer for everyone!) heading either east or west (TBA) and turn around at Burke Rd and the agreed spot at the other end.
Unfortunately,either of these "loop" versions would not have a single coffee stop at the end, but at that time of day there are other things I need to do, and the ride itself could be sociable enough that a good chat afterwards was not needed!
Canterbury Rd profile at http://www.bikely.com/maps/bike-path/Ca ... Rd-profile
Edit: you can click the elevation tab to see the hills.
Office use only.