You will drive by Humpty Doo on the road to Kakadu, or if taking one of the several jumping crocodile tours on the river. The rown itself is a very short deviation off the main road, and has a general store, pub, cafe. Reasonable spot to stop for a lunch, supplies or a beer. Not much in the town itself. Don't overestimate its size by the font on the map. It is close to several national attractions.
Along the Arnham Highway, less than an one hour drive from Darwin. There is a scheduled bus service from Palmerston in Darwin, but it is mainly a school service, and is unlikely to be convenient for a traveller.
In practice uou will need to have a car, or be on a tour to get around this area. It is hard to imagine anybody wanting to be in Humpty Doo for long with no transport.