The South Coast (that is, anything South of Bateman's Bay) is much, much cheaper than the rest, and it has all the charm of any other beach community with loads of unique attractions in its own right.
Although Magic Mountain is technically for kids, my husband and I visited twice before we ever had kids. We had a great time! The drive takes about three hours, so it's best to combine your fun with some great local food. I recommend
Wheelers Seafood Restaurant and Oyster Farm or the
Wharf Restaurant and Aquarium.
I also recommend buying a ticket with unlimited rides, so you can go on everything as much as you like. The correct sequence of rides is roller coaster, toboggan, water slide, toboggan, lunch, roller coaster, water slide – because you can't go on the roller coaster wet, but you can dry off on the toboggan.
The roller coaster would be extremely thrilling for primary and middle school kids, but teens would probably prefer the toboggan since you can set your own danger level (my husband and I both fell off at different times). Plenty of places have waterslides, but THIS one is largely underground which is excellent.

See the carefree joy in our eyes?

The posts are air holes