If you are particularly interested in seeing some outdoorperformances, you should head 1.5 km downstream of Zhongshan Bridge to the Retired Officials Park (老干部公园 Lǎogànbù Gōngyuǎn) west of the Huánghé Dàqiáo Bridge (黄河大桥).
This isn't the real name of the park (no one I asked seems to know it), but I've named it after the Retired Officials Activity Centre (老干部活动中心 lǎogànbù huódòng zhōngxīn) next door. In the afternoons it is usually buzzing with music performances of traditional local opera, ballroom dancing lessons, and games of Chinese chess or cards. There are masseuses, boiled egg sellers, newspaper vendors, as well as plenty of tables and chairs where you can sit, drink tea, and take it all in.
From the Retired Officials Park, you can continue downstream through the Children's Park (儿童公园 értóng gōngyuǎn) at the south end of Huáng Hé Dàqiáo Bridge (黄河大桥), and then onto the Waterwheel Garden (兰州水车博览园 Lánzhōu Shuĭchē Bólăn Yuán).