The old botanical garden on Pilgrimstein is one of the oldest university botanical gardens in Germany, established in 1809 on the slopes above the Lahn. It is small — 3.6 hectares — but beautifully maintained, with a good collection of medicinal plants, an arboretum section, and a greenhouse with tropical species. The terraced layout takes advantage of the hillside, giving views back over the river valley.
As a place to spend an hour, it's ideal — quiet, shaded, and largely undiscovered by visitors who are focused on the castle and the church. University students use it as a study spot in good weather. The garden is free to enter.