I'd have to second that. Wealth of knowledge. Thought I knew how to fish until I did a range of trips with other members from there, sometimes on my boat, sometimes on theirs, highlight being a 2 night/3 day voyage off the coast catching everything from tuna, macs, jewies,coral trout, red emperor and so on.
I won't bore you with the obvious, but its worth stopping off for a dip at Bitter Springs in Mataranka. You'll want a swim in October up there, and swimming holes are harder to find in Kakadu.
Also I imagine you'll turn off at Pine Creek, but if you did go a bit further up, a tour of Litchfield N.P. makes a nice relaxing day. Also, lesser known, are some good fishing holes there- PM me if you'd like details.
Kakadu land based, well, its not off the beaten track, back the old Cahill's Crossing is worth a cast. I caught me first barra there in short order.