You could try Williams down Albany Hway for lunch at the Wool Shed, great Lamb Shanks, then turn right toward Collie, good road, normal speed limit and a bit curley in places.
Cold drink at Collie then down Coalfields Highway and stop in at Wellington Dam for a drive around the bottom of the dam before heading back to the Coalfields Hway to either the South West Highway or the Forrest highway and back to Perth.
Hills all over the place, tree lined route and not too hot.
Been wanting to do it for some time but I don't know if I will be back from Ballarat by then.
Regards
HOFFY