I put together a blog about our trip to the PNG highlands and the Sepik River, and you're welcome to have a look:
It might be helpful to you in knowing what to expect. (Note that you can click on the circular photos to get slideshows. The rectangular photos are embedded in the text.)
We used Paiya Tours to organize our visit to Mt. Hagen at the time of the sing-sing and stayed at their Magic Mountain Nature Lodge outside of town, which was much nicer than being holed up in walled hotel compounds inside Mt. Hagen. Paiya Tours did a great job.
We also went to the Ambunti Crocodile Festival on the Sepik, and that was incredible - only about 20 tourists among about 800 spectators... you might want to consider that as an alternative or as an adjunct to a Highlands show. We had a great experience using PNG Frontier Adventures to organize our Sepik Trip. They were very well organized, security conscious, and our guide Pomat was incredibly good!. We found the Sepik to be the most interesting place to visit.
Both Paiya Tours and PNG Frontier Adventures were extremely good. Before deciding on them I spent a lot of time on this forum and found only positive reviews for both Paiya Tours and PNG Frontier Adventures, but mixed or negative reviews for many other operators, so I took a chance and went with those two operators... and it paid off. I highly recommend them. (You'll get lots more detail in my blog.)
The bottom line - a remarkable trip, especially timing it to attend one or more of the big sing-sings.