Personally, for 7 hours, I would not stray too far from DRW airport.
Yes, its a small airport, and nothing much to see and do around it, but if you can, just do the hotel day room stay at either of the two airport hotels.
Getting an uber would eat at least an hour, for the trip to Darwin city, and back, maybe hour and a half.
Even with linked PNR, and bags tagged all the way, people say Darwin can be an eye opening city.
If you had a day, then yes, do the city wave pool and etc, but for 7 hours, I would not go too far.
Plus Darwin is hot, and more humid than Brisbane, so you would maybe need to change clothes and have a shower after the Darwin city visit, so this would also add on time.
But thats me.
Of course, to each their own.