Must see/do things in Sydney with 3 year old and 6 month old

[BouncyTigger]

Off to see our best friends in Wollongong in 2 weeks for a month and looking for some top tips for things to do in Sydney for days out.

We will be visiting the Zoo and Aquarium, taking the ferry round the harbour etc. But does anyone have any other good tips - especially Christmassy ones as we tried to book at the Opera House for their Christmas toddler show but it was full.

Thanks

We went when DD1 had just turned three and she really enjoyed Luna Park.

Avalon has a really nice beach as is the one a little further along (the one where they film Home and Away). They both have sea water swimming pools which where really nice.