Actually, to be a bit more helpful, Horsham is about as flat as you could imagine.
Mount Arapiles (about 30-45 mins drive)is a huge rock outcrop and is the only thing around there that has any elevation. There are walking tracks around there that access various crags and go up to the summit.
It has been years since I've climbed there, but I would be reasonably certain that taking a downhill bike there would be the surest way to piss off the locals and Parks Vic as I believe they now control it. I believe there to be some tracks on the backside, but don't think they would be 'interesting' enough for you.
I'm happy to be corrected on this.
Your other option is the Grampians. It's probably about an hour away(from recollection) It's not controlled by Parks and may give you many options. Mount Zero (on the Western Highway)has some interesting terrain. Imagine something like slickrock in moab but with more features (ie crevices etc). I had always thought that it would be intersting to ride a bike there, but was always too afraid of death.
Probably not all that helpful but -shrug-
