Heiligenberg is the wooded mountain across the Neckar from Heidelberg's old town, and it carries far more history than the view first suggests. Celtic fortifications, Roman sacred remains, medieval monastery ruins, lookout towers, and the 1930s Thingstatte all sit within walking distance of each other.
The climb is a proper half-day if you start from the Philosophers' Way and keep going to the summit. It is one of Heidelberg's best escapes from the crowded castle route, with enough shade, archaeology, and river views to justify the effort.