During our Darwin stay we got a few tips on other, not as much well known, spots we should check out prior to departing south/west, namely;

Gunn Point

Gunn Point is a forty minutes north east drive of Darwin, via a dirt road to a reserve where you can free camp on the beach. The main attraction is simply relaxation............and fishing

Dundee Beach

Dundee Beach is on the opposite side of the Stuart Highway and is a bit further away from Darwin (approx 1 hour 40 minutes), but again is a free camp on the beach, where the main activity is relaxation and fishing.