Hi there I wrote a post on what to do in Sydney in a day, will try to find it. I presume you have a few days there so could take in more.. The drive to Melbourne you could do in 10 hours with a break but best to do it as you are and stop in a few places break it up. There's some beautiful beaches, it's April so nearly winter the water temperature will be low but okay for a dip. Good website here :

www.australia.com/en/itineraries/nsw-sydney-to-melbourne-coastal-drive.html

I use to live on the edge of the national park and wattomolla /bundenna, as you leave Sydney 50 mins drive got through national park and check out wattomolla beach, amazing spot.

www.tripadvisor.com.au/Attraction_Review-g255333-d7148345-Reviews-Wattamolla_Beach-Wollongong_New_South_Wales.html

After there take the coastal road down to Wollongong. After the 'gong' there are beaches all the way down to Melbourne. Kiama is nice stop off I would definately stay in jervis bay, amazing beach and national park and the Huskinson pub in the evening.

www.tripadvisor.com.au/LocationPhotos-g528972-Jervis_Bay_Shoalhaven_New_South_Wales.html

And hour from there is mollymook, we loved going there and been hammered in the surf club a few times with the locals. Another hour down the road after mollymook is Batemans bay, great beaches, bars etc.. All the beach towns will start looking the same. Not sure how long your taken to get down? But Melbourne has a lot to offer also, love the city, get into the cookie bar it's around the corner from federation square and hosier lane (dedicated to street art) your Melbourne family will have an agenda for you and a trip to ocean road etc.

Anyway hope it goes well and drive safely. The trucks are not like UK some are 20m long on a long drive it can feel like spielburgs film duel

Thanks for the ideas Jeezuz Jones Snr and @sevendeadlysins I have done a fair amount to research but it is great to get first hand from people in the know. Cookie bar looks cool, I am staying just around the corner In Melbourne so will pop in. Ill be in Sydney for about 4 days so plenty of time to check out what it has to offer.

Would love to see Coober Pedy but its is really in the middle of nowhere. Really do need more than 3 weeks to see Australia

Keep the ideas coming please

Take the coastal road and not the route through ACT. Loads of little towns and picturesque scenery and head to Philip Island for the Penguins dude. I did mine the other way, started in Melbourne and took a 3 month drive to Cairns. Amazing place.

You'll just miss out on Vivid in Sydney, which is a bit of a shame.
Do the touristy stuff first. So basically, go to Circular Quay, walk around there to see the Habour Bridge (walk across it, it's free), check out the Museum of Contemporary Art, Opera House, The Rocks. Then catch a ferry to Manly where there's a nice beach, grab some fish 'n chips and a beer and have a wander. There's board hire there, so you can try your hand at surfing. But the water might be a tad cold.
If you want to escape Sydney, you can catch the train or a bus to the Blue Mountains to check it out. Some amazing views and it's very peaceful.
That's about all I can think of at the moment. Any specific questions or ideas you wanted to run by, just ask