Sorry I didn't see this sooner Rien. I'm in Melbourne and can help you out a bit.AWESOME! ... thanks a ton, especially for the St Kilda info ... look forward to your suggestions/recommendations ... I still have another week or so before I travel to Melbourne. Cheers.
If you are worried about getting shots of penguins at Phillip Island you can always ge some at St Kilda. There is a small colony there and you can get quite close. Photography is permitted so long as you don't use flash.
I have to go out now but I'll write more later
I should have added that seeing the penguins is free at St Kilda too Actually you could make a pretty good time of it there. Have some of the local favourite fish and chips while you wait for the penguins to arrive dusk - we call the fish flake but it's really from gummy shark. Your trip falls into the 2 months of the year when the sun hits Eureka tower at the right angle to reflect light like in the image attached. I took it while I was waiting for the penguins a couple of years ago. Luna Park is also nearby and has the oldest continuously operating wooden roller coaster in the world.
I see you are planning to go to all 3 zoos here. Have you considered a zoo membership? It would work out to be $173 for the family. Otherwise its $30 per adult at each place plus $13 for each child 4-15 on weekdays (children get free entry on public/school holidays and weekends here). Check zoos.org.au. Make sure you take in the flight arena show at Healesville - they have a wedgetail eagle that flies low around the arena.
I'm sure your kids will love to see the Myer Christmas windows in Bourke St Mall but beware there can be long queues during the day - better off seeing them around 5.30-6pm when most people are preparing to eat at home.
As others have mentioned most of the camera shops are on Elizabeth Street. Michael's is the largest and of course has the largest range but it also has a camera museum with some interesting items to see. Ted's is smaller but ok. They will both price match but only with other Australian stores. Digital Camera Warehouse usually has the best prices here but they are about 10km out of the city. Check their website for prices to use for a price match if you do really need to buy something;)
Hosier Lane off Flinders St is the main place for Graffiti. Also check out Degraves St and Hardware Lane for some street eating scenes. The Block Arcade and Royal Arcade are historic shopping arcades that offer great architectural shots.