Hi there,
Yeah definitly spot the Arnhem Highway from the Fog Dam turnoff for about the next 15km. You will notice that you drive through floodplains. These are great for seeing Floodplains Rats, Watern Pythons, Keelbacks, Slate Greys, Floodplains Adders, Mulga's and if you walk the side of the road (if the water comes right up) Mcleays Water Snakes.
Also WALK the Fag Dam wall (its a road) and spot with a torch looking on the sides and in the water. You will see most of the species listed above bar Adders. The general area around Fog Dam you will also see Darwin Carpets on the roads if you are lucky.
Litchfield NP near Batcholor is awsome to spot as well. You will see Western Browns, Hill Adders, Carpets, Olives, and other smaller species.
The road from Katherine to Timber Creek is great for Childreni, BHP's, Mulgas and Geckos.
During the day you will see Frillies everywhere if conditions are right ie its started raining