If you stay out of the Highlands you could do a solo mission. Horn Island to POM via Daru or non-stop and then coastal to Alotau, China Strait and Losuia for the famous crabs.
Then head to Tufi for some diving and continue to Lae/Nadzab. Refuel there and go via the Markham Valley (very wide) to Madang and continue coastal to Wewak and Vanimo.
If the weather is good then head over the hills back to Kiunga and then to Horn Island.
From Lae/Nadzab you can also reach the New Guinea Islands which are also very picturesque - the hills aren't that high either over there.
Nice places for diving and fishing are Kimbe (Hoskins), Rabaul and Kavieng. There is also a fishing resort down in Gasmata but I am not sure if that is still going.
If you wanna head up the Highlands I would ask someone at the MAF to tag along for a donation. Goroka, Mt Hagen and Tari are some good places to visit; especially during their annual shows.
As it has been pointed out before - AVGAS is the biggest problem, NVFR is not permitted in PNG and STAY OUT OF CLOUD at all times!!!
Enjoy and lukaut long antap (be careful in the Highlands)
AB
P.s.: If I had the time I'd come with you - maybe when I am retired
P.p.s.: I get a bit emotional typing this post - I had the time of my (GA) life in PNG.