Autumn in Sydney, will be a good time for photo opps... hmmm... 3 weeks huh. Apart from the usual tourist attractions around the Harbour, can consider going for an early morning balloon flight above the Hunter Valley, opportunity for some great views over the Hunter Valley - after that you can visit the many wineries there (also great photo opps). Katoomba and the Blue Mountains and the Jenolan caves are another good option. Or you can go to Jervis Bay for the nice beaches and the dolphin watching, or also to the Fitzroy Falls area for the nice waterfalls and the rainforest around there. You going with the family ? Taronga zoo is a good option for the kids.