If you've made it all the way to Hongjiang, don't overlook nearby Qianyang, another historic river town with a completely different architectural style. In contrast to Hongjiang's narrow alleys and high walls, Qianyang consists of wide-open flagstone streets, fronted by wooden clapboard buildings (most of which date back to the Qing, although there is also one Ming house) and grander yinziwu-style compounds that once belonged to noble families (characterised by a series of adjoining courtyards, high exterior walls and concave roofs).
You can visit the town on a self-guided tour (map provided), passing a number of historic buildings open to the public. Following the tour will eventually lead to the red sandstone West Gate on the opposite side of town, which opens onto the Wu River and Qianyang's most famous landmark, the Hibiscus Tower. Perhaps less impressive than the town itself, the arcaded gardens surrounding the historic structure are nonetheless worth a wander.
Qianyang is in the town of Qiancheng (黔城, Qiánchéng), which can be reached by bus from both Huaihua (¥14, 45 minutes, half-hourly, 7am to 6pm) and Hongjiang Old Town (¥6, 30 minutes, half-hourly, 6am to 6pm). Buses 1 and 2 run from the Qiancheng bus station to the old town.