You can't drive your car to the Eagles' Nest. Access is via buses which go up a private, closed road. There is a parking area for cars in Obersalzberg at the base of the Kehlstein (mountain on which Eagles' Nest, actually called Kehlsteinhaus, sits). A RT ticket for the bus and the elevator from the bus parkplatz to Kehlsteinhaus will cost you €15,50 per adult.
In Obersalzberg there is a museum, called the Dokumentation Center, about how the Nazis gained control. There you can also tour the remains of the extensive bunker system that used to connect the SS base there with the vacation homes of all the top Nazis in Obersalzberg. It's easy to spend more time in the museum than at the Eagles' Nest.
Or, if you have had enough of Nazi history, drive down to Schönau am Königssee and take a boat ride on the lake.