I don't think there's anything particularly exciting to see on the drive between Sydney and Melbourne... I'd say that flight would be A$140 well spent - it'll give you an entire day (or night) back.

If I didn't have friends in Sydney, then a week might be a hair too long if I were limited to the metro area. But there's at least a day's worth of fun in the Blue Mts (great for light hikes) - that's 2 hrs' drive from the city. The Hunter Valley is another good excursion if you're into wine and winemaking. And it might not be everyone's cup of tea, but a drive to Canberra could be fun too. (What can I say - I'm a political junkie, and the national museums are superb too.) That's on top of all the well-documented things to do in the city - BridgeClimb, the zoo, the art museums, botanical gardens, cliff walk, etc.

As for Melbourne, a week might go by even faster. See the penguins on Phillip Island. Mornington Peninsula and Yarra Valley both great for foodies and oenophiles. Dandenong Ranges for nice hiking amid tall trees and rainforest. Fantastic museums. Sporting events galore...

And your Tassie week is already planned for you.

So yes, you could have a completely non-boring week in each place, but if that wouldn't suit you, constitutionally, then definitely keep the side trip plan. You gotta be you. If you still want to fill a 3-day Big Empty Space, maybe the Great Ocean Road would be your ticket? It's sort of adaptable and you could do a driving itinerary of 2, 3, 4 or 5 days round trip from Melbourne...