For dinner tonight we recommend going to the pier at the Darwin Waterfront. You'll find a series of stands that sell fresh fish prepared in a variety of ways. We recommend the barramundi (typical regional fish) fish and chips at any of the stands (they're all equally good). It'll run you about 7 AU$ per person. Note the fish sticking their heads out of the water waiting for people to toss them French fries!
To get there, take the car to the Port of Darwin following the signs. At the end of the pier is a parking zone. Leave the car here and walk to the stands.