Above are some great ideas! I was going to suggest Aussie World, it's closer to where you will be...great for kids & much cheaper to get into than the others. We are yet to get to the zoo, put that on the list for sure. The beach, free & simple....kids love it. We stayed up there last Christmas & rode to the beach everyday, took up the morning well & then DS had a great arvo nap.
And don't forget about yourselve's, kids don't need a lot of money spent to have a good time.....just a park & change of scenery & they love it. Ahhhh help me out, what are the name of the markets up there???? they are awesome also!