I wouldn't do East Gippsland as a day trip - too much to see and do and it takes a good 3 hours at least to get there from Melbourne. I don't visit my parents unless I can get in two nights minimum. It's such a mindnumbingly boring drive that I have to have that break in between!
Meanwhile, once you're there, that whole region is a goldmine
I've just been reading my copy of Gippsland magazine and I've got some ideas for heading out for one of the nights with just DP and I, so that the kids stay with my folks till bedtime. Bruthen is a town I have been dying to explore (we only ever drove past it to get to Buchan as kids). Metung is a nice place but a bit overrated, I feel (and my parents lived there and still have a vacant block there). Rail Trail, wineries, kayaking, boat hire on the lakes, fresh seafood at Lakes Entrance...I do love that area Not a day trip, though.