Fraser island

Do a search and you'll get a million similar threads.
Take some bushmans to help with the march flies, if they are around.
Sun screen. Cold drinks!!

Drop your tyre pressures to 18psi, or whatever is your choice, before you cross the beach at Inskip Point to the barge. If you can get through that chewed-up sand you will have no trouble on the island.

Be sure to obtain a tide table and avoid travel within 2 hours before or after high tide. Keep below 80kph and watch for creek crossings on the beach.

Being school holiday time it will be busy with people on the beach, especially children, so be alert for them.

It will be hot, so take hats and sunscreen and plenty of water to drink. Drinks, food and fuel are available on the island but are expensive.

Have a great time mate.

Make sure you have time to see the entire island. It is a big place.
On the inland tracks watch out for the tour busses, a few think they own the island and fly along.
The barge from Inskip Point (via Rainbow Beach) is the best way to get to the island, and at or close to low tide is best. You can get all of your permits at Rainbow Beach and there is no need to book your barge, just turn up and like Allan said, lower your tire pressures. The road to Inskip Point is sealed up to the point. That is where you need to lower your pressure, at the end of the bitumen.
Fraser Island is still a wilderness area so watch your children at all times. Dingoes are a problem; just enjoy them from a distance. I repeat, watch your children.
The beach is classified as a road, so you need to obey all road rules.
Central Station is a must and there is a great walk that follows a creek. It is possible to leave people to do the walk and someone can drive to the end to pick everyone up.
Have fun, the island is brilliant and very unique!
Kevin