Well you have 3 weeks, so that is a reasonable bit of time, have you considered doing the coast?
Say....start off at the top, eg Port Douglas/Cairns, then work your way down-->
-From Port Douglas you can skip into the Daintree area, Cape Tribulation, get into some snorkelling/scuba diving (if inclined) on Great Barrier Reef.
-Take in an island or two, ask around, read up on the islands, you might be a Green Island type of person, or Lizard Island, it's hard to say for another person..
-From Cairns there is Kuranda scenic train, sky rail back down from the little village up in the mountains, (Kuranda). There is inland from Cairns, beautiful waterfalls, small communities...when we "do" QLD there is always the highlight of the variety of wonderful people, the food, the scenery, the things to do...
-Say, from Cairns to Mission Beach, it's beautiful, again the people, food, scenery, fun-whatever you make of it, check out internet for things to do in Mission Beach, (that is where they film Sea Patrol).
-To Townsville, again, it is what you want to do, what u consider fun, gorgeous pubs and rest'ts there, catch ferry over to Magnetic Island, camp over there for the night, hire yourself a scooter and drive around the island.
-To Whitsundays, make Airlie Beach your base, all that ocean and islands to pick and choose, sailing, diving, snorkelling...(remember u are in stinger season too, plus high holiday season).
-If u want go from there to Mackay, do the Eungella Park if u are into parks and platypus's
-Go to Maryborough, Hervey Bay, head over to the world's biggest sand pit (island) Fraser Island...
-Head toward Brisbane, stop off at Scarborough, north of Brizzy and head to the boat harbor (Bird of Paradise Parade), check out the fish market...amuse yourself in Brisbane
So that is possible in 3 weeks, we have done it, car, motorbike, but better if u can mix with some flights..eg fly from Townsville to Mackay or Proserpine..
If you want to go way inland, that is Mr OO's speciality, he knows the way around inland, I'm more coastgirl.
Thing is you can have fun and adventures anywhere there, I'm really fond of the ocean, the reef, the parks, the nature, the food, the people...but you might like nightlife, bungy jumping or whatever it is...
Have fun.