The Kellerwald National Park is easily accessible by cars from the outside but you can't drive through it. A bike is the best way to explore the forest and some great species like Collared Flycatcher, Grey-headed Woodpecker and Black Stork can be found. It really gets amazing when combining this with a evening/nightdrive where Tawny Owl, Eagle Owl and even Boreal Owl and Eurasian Pygmy-Owl can be relatively easy to find on the right spot. It is recommended to do a location scout first to find the perfect spots for the species. In this way you can be at the right time for every species at the right spot.
Most of the car parkings (but not all) are on the north side of the park These parkings are mainly used by people that are visiting the lake. But most parkings are also connected to the trails. The longer trail that starts at the NP visitor center is a good place to start searching for Boreal Owl. There is also the option to bike thru the park. It would be wise to go for a mountainbike for most of the bikepaths since most of them are non-paved. In total there are 20 circular hiking trails from 13 hiking parking spaces. On the map you see the 5,5 km Hagenstein-trail that starts at the visitor center.