Heaps to do & see without 4WD.
Jim Jim falls will need 4wd - but most rentals there most likely not covered. But you can arrange a short helicopter ride there.
Don't miss Gunlom falls bottom end of Kakadu. Also Berry Springs just out of Darwin.
Heaps to see in Litchfield - only really need 4Wd for Sandy Creek & Surprise creek (but possibly day tours there).
Also Oyster Bar on waterfront for delicious Coffin Bay oysters & a cold bevvy