Directly opposite the Temple of Heaven and rarely on tourist itineraries, the former Temple of Agriculture (Xiannong Tan) houses the Museum of Ancient Chinese Architecture — a genuinely interesting collection of architectural models, building components, and documented historical structures spread through the temple's surviving halls. The temple itself, where emperors came annually to plough the first ritual furrow of spring, is beautiful and almost entirely crowd-free.
The museum's scale models of historical Beijing buildings allow a clear understanding of how the city was structured before its transformation in the 20th century. Entry is cheap. Combine with the Temple of Heaven — they are separated by a road and connected by a pedestrian crossing — for a full morning in the south of the old city.