There're usually information booths around I think,those should help you figure what to do.
Loadsa ppl have mentioned loadsa stuff,so I'll just add MovieWorld into the list,or specifically the Superman Escape ride there. I mean,even if you're not too big a roller coster fan,you might as well experience it since you're there

It's at Helensvale iirc,not too far from GoldCoast. (U can catch a bus frm GC)
Erm,are you into wine?

Loadsa wine valleys around,but not too sure if it's viable for you to travel frm the places you're going. It's somewhere north of Adelaide iirc. Barossa valley and what nots. Good food and good wine there

Must try food...kangaroo steak? Haha seriously,when in the right hands,it can be pretty good

If you're into steak,look out for Hog's Breath. Not a fancy restaurant,just some cafe branch thingie,but they have awesome primes.
If you're around Surfers' Paradies (GC) you can eat at SeafoodMania. (just ask around) Kinda like fast seafood thing. Decent price,nice food,large portions