Nitmiluk (also called Katherine Gorge) is the deep path cut through the sandstone by the Katherine River, and the Nitmiluk Katherine Gorge National Park is where you can go to lap up the luscious experience of the Gorge, whether that be swimming in it (sometimes with harmless freshwater crocodiles), canoeing in it, hiking around it, gazing it from an observation deck, flying over it on a helicopter...or any combination of the above.
The park is run by the traditional owners, the Jawoyn, in conjunction with the Australian government. It's a well-appointed place with lots of visitor facilities (and lots of visitors, especially in the dry season). You can choose your level of activity, from lounging around at your campsite or the visitor center café to strenuous canoeing trips or hikes. But make sure you take at least one long hike, perhaps to see the Aboriginal rock art, or at least to get sticky enough to make cooling off in the river a delight.
Nitmiluk (Katherine Gorge) National Park is swarmed by visitors in the dry season (May to September). In the wet season (November to March), the river goes into spate and swimming and canoeing can be restricted, as can access to the roads.
The park is about 30 km (18 mi) northeast of Katherine, which in turn is about 300 km (180 mi) south of Darwin on the Stuart Highway.