Kakadu National Park in the Northern Territory, 125 mi/200 km east of Darwin, is one of the few places in the world that is listed twice as a UNESCO World Heritage site: once for its amazing ecosystem and a second time for Aboriginal rock paintings.
With 1.2 million acres/486,000 hectares of wilderness, the reserve has swamps filled with crocodiles, innumerable species of birds and other wildlife. More than 1,000 Aboriginal paintings adorn park cliffs. (And if that's not enough, the park was also a filming location for the movie Crocodile Dundee.)
Hire a boat to tour the billabongs (lakes), which fill up in the wet season and shrink during the dry. It's best to visit from early March to mid-October; many roads are impassable during the wet season, around Christmas. No matter when you go, don't forget to take along insect repellent, a hat and plenty of sunscreen. http://www.environment.gov.au/parks/kakadu.