We did Katherine, Edith Falls & Leitchfield NP last August and really enjoyed it. I had a hip replacement some years ago and walked the round trip to Edith Falls. We used Wikicamps to locate camp spots and also Fuel Spy to compare the local Diesel prices. Both on an iPad.

Stay at Mataranka or Bitter Springs and swim in the pools.
Edith Falls definitely worth visit/stay.
Pine Creek area WW2 related things worth seeing.
Litchfield - must do. Good CP's Batchelor area, but longish drive to the best parts of park. More basic CP just to west of Wangi. Or try NP campground at Wangi -esp if you can get there early-ish in morning, after day's travellers have pulled out and before most of the influx! Lots to see at Litchfield and some walks you could do - been done by The Driver with his replaced hip: around Buley Rockholes, to top of Florence Falls, to Tolmer Falls LO, Greenant Creek walk, Wangi circuit walk, Walkers Creek walk. Swim at Wangi and Buley Rockholes. If you can do gravel roads, can drive the direct way from Wangi area, through Woolaning, to join the bitumen a bit west of Berry Springs, and thence to Darwin. Could visit Territory Wildlife Park while on that road - well worth doing.
Heaps of things to see and do in Darwin, of course. Howard Springs CP is a bit out of town, but not as dear as ones closer in. Do not miss - Darwin Museum & Gallery, for Cyclone Tracey feature and aboriginal art displays; wharf precinct - great meals; Indo Pacific Marine display; WW2 oil tunnels; the old historic end of the central part of town; check out if Star City Casino & hotel has cheap seniors lunch on I think Tuesday - used to be great value; ditto check out Yacht Club meals; Mindil Beach Markets on Thursdays; Parap Markets on Saturdays - more fresh produce there; if you can afford it and want a tour with a difference, check out the day trip offerings to the Tiwi Islands.
From Darwin can do a loop through Kakadu to rejoin highway at Pine Creek - saves backtracking, road all sealed.
On way to Kakadu is Window on the Wetlands. Aslo Jumping Croc cruise - but you will see plenty of crocs in a more "natural" way by doing a sunrise or sunset cruise on Yellow Waters at Cooinda. Go to Jabiru for overview of the park etc, then go up to Obirr for rock art - NP campground. After that could camp Nat Parks or commercial (Jabiru, Cooinda). Visit Nourlangie Rock rock art galleries, do Yellow Waters cruise mentioned. I believe that there are very good heli flights available over the kakadu area, but have not done one myself - reckon would be very scenic.  Visit Maguk gorge, camp at Gunlom.

And all that should occupy you for quite some time!!

-- Edited by wendyv on Thursday 9th of June 2016 05:56:58 PM

Thanks very much for your comments.
We've got wiki-camps on the wifes mini-ipad and use it a lot. We had a petrol price program (Fuel Map), but found it wasn't updated very frequently and always needed an internet connection, perhaps fuel spy is better; we'll give it a try.
I tried looking for Olloo Crossing; is it on the Olloo road? As this appears to be very close to Litchfield NP and there seems to be more to see there, we'll probably have to give that a miss; but thank-you Haji-Baba.
I almost need to print out wendyv's reply and tick off each suggestion; thank-you wendy for your very detailed answer; and thanks also for your camping suggestions in Darwin. We are going to leave Daly Waters tomorrow and camp at Mataranka.
By the way; the Daly Waters pub; there's a field adjacent to the pub, it's packed full with caravans, there's an "overflow area" over the road which is also packed, we're down the back of the field in an area reserved for campers/camper-trailers, but its so full there are caravans here too. The pub has a huge beer garden with live entertainment. They promote a "barra & steak" meal with a salad bar for $31. We didn't partake, but it was reportedly very nice. Unpowered camping is $9 per person per night. They say there's a pool, but you have to walk through the bar to get to it. I guess a lot of people like this sort of thing, we met a couple that have been here twice before and have come again! I can't dispute the attractiveness of this place for some people (a lot of people based on the number of vans here), but we're a bit more into crowd avoidance. We were going to stay 2 nights, but 1 night is enough for us!