Kakadu - it's well worth a visit if you are in the Northern Territories, and is an easy couple of hours drive from Darwin.  4WD is not necessary to see the main famous sights.

 The Aboriginal rock art at Ubirr and Nourlangie are definite must sees.  The rock faces are covered in ochre paintings of fish, kangaroos, people and spirits.  The views from the top of these rocks are worth the short climb (even when it is the hotter times of year - honest!).  Both of these areas were also film locations for Crocodile Dundee - if you are a fan, pay particular attention when you are standing on the top of Ubirr, do a 360 degree turn, and you will recognise the view from the film.  Another location from the film is Anabangbang billabong.  Ubirr and Nourlangie have good road access, parking and toilets.  There are a number of options for seeing the art, particularly useful if you have mobility difficulties.

Another fun activity is to take a flight over the national park and over Arnheim land.  Flights go from Jabiru airport, by the uranium mine, and are a good way to see areas that are difficult to access, and often require permits to enter (Aboriginal owned land).  It's lovely to see the waterfalls, and wetlands, as well as the very rocky landscape.

Jabiru is a reasonable base to stay to tour the park, and there are a couple of different options to cater for different budgets.

Get yourself on a tour, or drive up to Kakadu, and you certainly won't regret it.