Possible Australia Holiday

[Shermanator]

I'm considering going on a holiday of a lifetime to Australia for around 3 weeks over Christmas and New Year 24-25. I would be in Brisbane and Sydney. Any suggestions on things to do? Also, I haven't been on holiday abroad for over a decade, my last holiday was 5 days in Cornwall in February over 5 years ago, so any tips on how to get the best deal when booking?