Fraser Island wasn't my favourite place on the East Coast. It was worth a couple of days though - make sure the weather is going to be nice, and find some other people and see it at your own pace by hiring a 4wd - I did an organised tour and regretted it, it was a little frustrating.
Don't stop at Cairns! Head on up to Cape Tribulation for some proper rainforest.
Also, sailing in the Whitsundays is a must, even though everyone does it, it really is a bit special, mmm.. Whitehaven beach.
I actually enjoyed Mission Beach too, though it seems to divide opinion somewhat, I thought it was nice and laid-back.
Brisbane is a pleasant and friendly city to spend a day or two.