The Orangerie is a long, low Baroque pleasure palace stretching along the bank of the Fulda at the edge of the Karlsaue park. Built between 1703 and 1711 by Landgrave Karl, it now houses the city's astronomy and physics museum (the Astronomisch-Physikalisches Kabinett) and a planetarium.
The Karlsaue itself is one of the largest Baroque parks in Germany, an island-and-canal landscape of formal flower beds, mature trees, and meandering paths along the river. The setting includes a swan island, an artificial canal system, and during documenta years a whole second exhibition site spreads through the park. Free, always open, perfect for a long evening walk.