Been to Australia a couple of times, I was in Darwin for a month in 2001, Fantastic place, if you are there on holiday. Gimlet has pretty much covered it. Plenty to see and do, from Fannie Bay Gaol to Mindil Beach market, If the Tour Tub (it's a mini bus) is still going it's worth it, you pay once and can get on and off as many times as you like. When I was there you could swim in certain water holes in Kakadu and Litchfield National Parks, but it's best not to now without local advice. I swam up to Twin falls, it was the only way to reach the pool at the base of the falls, but it's now too dangerous due to crocs, I think there's a boat that takes you up now. Best to buy a good guide like Lonely Planet. you should have a great time.