I have been here several times with expedition ships. It was one of my favourite stops in New Guinea, because it was one of the few places we could do a nature walk, and because of the open habitat, one could see flowers, butterflies and birds quite easily.
The Dei-Dei Hot springs are located on Ferguson Island, part of the D'Entrecaustax islands in south east Papua New Guinea.
There is a trail from the main village. It goes past small huts and nice gardens. The trail leads though open Eucalyptus woodland, Pandanus woodland, and then into Melaleuca open forest around the springs. So there are many plants and animals similar to what is found in northern Australia.
Many of the flowers (even the weeds), particularly along the trail edge, attract butterflies and other insects. Around the hot springs there are quite a few Pitcher Plants, carnivorous climbers.
The Curl-crested Manucode is one of the main birds to try and find here is, as it is a bird of paradise and endemic to this groups of islands. It is relatively easy to see in the tops of the trees around the open Melaleuca woodland. Unfortunately it is rather more crow-like than the image of a bird of paradise you may have in your head. However, if you see them in their energetic display calls they are fun to watch.
(and they occasionally turn up as pets in the nearby village, as below...)
Other common birds on the track are Yellow-bellied Sunbird, Eclectus Parrot, lorikeets, finches, honeyeaters, etc.
The hot springs themselves are used by the locals for washing and cooking. Obviously there are no gaurd rails here, so be careful. There are local guides, and it is important to follow where they step.