Melbourne

Bit more of a serious answer:
It depends on when you are in Melbourne. Winter is miserable weather-wise by Perth standards. You can literally go a week without seeing the sun. Melbourne has adapted to this by gearing its infrastructure towards the indoors.

1. Wander the alleys:
   From Southbank, wander over the Yarra, check out Flinders St Station in passing, then cross to Degraves St. Have a coffee somewhere or find a nice treat to eat. Make your way up Centre Place, The Baguettes in here are amazing if it's lunch time. Keep going through to the arcades, you'll eventually get to Bourke St.
2. Trams in the city are free:
   Hop on a tram up Bourke St to Spring Street. Up here, you can check out Princess Theatre, The top of Chinatown, Parliment House (Once you know that building, you'll recognise those steps in any new story about Melbourne). Go check out Treasury Gardens next door. There's a memorial to JFK there that even the average Melbourne local doesn't seem to know about. If you are not walked out, you can keep walking through Fitzroy Gardens, and jsut past that is the MCG.
3. MCG:
   Even if you don't go inside the MCG, it's worth walking around and looking at all the sporting legend statues around the stadium. If you are free for a weekend though, I recommend checking out an AFL game in there. They do tours through the week also.
4. Street Art:
   If Street Art is your thing, you've probably seen a lot by now in your wanderings. The center of it in Melbourne is Hosier Lane - on your way from the MCG back toward Flinders St/Southbank.

All that should keep you busy for half a day. Other things worth considering:

• Botanical Gardens.
• Melbourne Museum.
• State Library (Ned Kelly's armour is usually here).
• Queen Vic markets on market days. Some nice food in here.
• Not Docklands. No idea why the visitors centre keeps sending tourists to Docklands, honestly there is nothing that interesting there.
• Check out what's on. See whether there's a show you want to see. Maybe there's something cool at IMAX? There's usually something going on.

Puffing Billy, and if you have small children... Diamond valley railway

Penguin Parade at Phillip Island might be a bit far (you'll need a car) but I think it's worth visiting. The Little Penguin subspecies is only found around southern Australia, Tassie and maybe New Zealand ☺

Here's an article on it.

I really enjoyed Scienceworks and the Melbourne Aquarium as a kid. Not sure I'd go far as to call them must-see, but worth checking out if your crew is keen on them!

Melb museum is worth a look, and there are things for all ages

You can go to Docklands and see the Melbourne Star!! You can't ride in it, but you can see it!