Funny. I don't know what to suggest as flights within Australia can be expensive.
We did Tassie last year, I remember going at 14 with my parents and enjoying it. We drove our own car and caught the boat so it depends where you're located if that is an option.
Margaret River and Broome are two of my favourite places.
I love the Sunshine Coast.
Questacon is a great place to go and cockington green is near there as well.
Tangalooma on Moreton Island. I've been going since I was a kid and love it. I've never been in September though.
You could go camping in a National Park or at one of the Big 4 ones. They have powered sites and activities for kids.
"Insert witty signature here"
I second Bali - but not sure where you are so hard to comment, flights could be prohibitive but from Perth it's ideal and so cheap, warm, welcoming and any kind of holiday you can imagine for yourself - once you get there... We just got flights for the three of us, return for $710 including luggage.
Have a look at the Big 4's as they're around the country and totally geared up for families, but not so in the annoyingly, crazy, loud family way!
A holiday I never forgot as a kid was a trip to the snowy mountains. It was pretty budget, we stayed at a holiday park at Cooma and travelled up to thredboe or Jindabyne each day, and as I had never seen snow, had a ball.
Don't know about that time of year though, or how relaxing it would be lol
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