Have sailed a few of the lakes down there as it's part of my territory with work and have snuck the kit along with me when the wind forecast looked good!
Lake Poorarecup east of Frankland was a fairly good sail with strong and steady breeze. Had a great couple of days sailing on camping trip with the family - especially just before dusk. Camping is free (I think - never charged us on any of our four trips now). General store about 10km away (sells most things inc alcohol!).
Lake Nunijup (closer to Cranbrook), although not as big as Poorarecup, has a flatter landscape and cranks in a decent breeze.
Lake Towerining near Darkan looks good too, but never sailed there. big lake, with powered sites.
I have been to Lake Dumbleyung about 8 years ago when it nearly dry. Would love to see it full now!
Website attached from 2004 but lake is some 13km long & 6.5k wide!!! "Far Canal!!!!" They have a yacht club there for the
locals.
www.smh.com.au/lifestyle/dumbleyung-20040208-gdkr01.html