Following informations are taken from link below which is very helpful as page is written in English as well: In contrast to other forested areas, northern Kellerwald Forest is characterized by its unusually high percentage of older broad-leafed trees. In hardly accessible locations along the Lake Edersee slopes, even remnants of genuine ancient forests of original beauty still exist. Old stands of beech and oak form the essential livelihood for numerous large bird species, bats, and wood-resident insects and fungi, all of the six woodpecker species of Hesse are found here, as are various storks, owls, bats, and numerous beetles, too many to list them in all of their varieties.
Pls take also a look at vichatherly's Eder Dam page
Close to the village of Hemfurth, a new attraction lures hundreds of visitors to the Edersee lake: The so-called Treetopwalk. This is a large ramp that leads you up into the tree crowns where you can learn about animals living there and the lives of trees in general. What might be an interesting experience is spoiled by the incredibly stupid signs that seek to entertain little children. Adults cannot but laugh when they read things like " Many animals live in the tree crowns. Among them is also the: forest bug." Every sign is exactly alike and (as in my example) often full of typos. Show me the child that is actually entertained by that...
Apart from that, the Treetopwalk is alright - you walk along some ramps some 15m above the ground which has - ironically - been cleared of most trees in order to make room for the Treetopwalk. Now and then, there's a nice view over the lake. Sometimes you even get a feeling to be in the tree crowns, but not very often. All in all, it's not worth the money (5 Euros).
Unique Suggestions: Apparently, the Treetopwalk company also organises dinners high up in the trees which might be more interesting. Further information can be found on their website. At 99 Euros per person, however, this is quite expensive.
Fun Alternatives: Edersee lake is good for swimming and sailing, and around the lake there are many cycling trails. Nearby is Waldeck Castle, which is definitely more interesting.