Crocasaurus Cove
http://www.crocosauruscove.com/ is pretty good.
Darwin Aviation Museum
http://www.darwinsairwar.com.au/
Darwin Harbour or Mandorah Dinner Cruise
Berry Springs
Fogg Dam
Aquascene (Fish Feeding)
I know they don't necessarily meet your criteria, but they are the one's I could think of.
I see that you've done it, but Litchfield is always worth the effort.