There are of course a number of venues to consider:
Huguang Guildhouse (Huguang Huiguan 湖广会馆)
The ECCS trips include a fieldtrip to the Peking Opera to this historically significant location. The Huguang Huiguan in Xicheng District, south east of Tiananmen Square. Take Subway line 4 to „Caishikou“ station and walk 450m east. Or take Subway Line 2 to „Hepingmen“ station and walk south for 550m. Prizes range from RMB 380 to 580. What you see are parts of various operas, such as the „White Snake“ (Baishe 白蛇), „Farewell my Concubine“ (Bawang Bieji 霸王别姬) or „Journey to the West“ (Xiyouji 西游记).
北京市西城区虎坊路3号, No 3 Hufang Rd, Xicheng, Beijing (map), +86 10 6352 9140
If you want to see full operas (plan 2-3 hrs) then you might want to go to these places:
Mei Lanfang Grand Theatre (Mei Langfang Dajuyuan 梅兰芳大剧院)
This major theatre in Xicheng District was built in honor of Mei Lanfang (梅兰芳), China’s famous traditional opera star. It is right on the 2nd Ringroad near Subway Line 2/Line 6 „Chegongzhuang“ station.
西城区平安里西大街32号(官园桥东南角), 32 Ping’anli Dajie, Xicheng, Beijing (map), +86 10 5833 1288/1388
Zhengyici Peking Opera Theatre (Zhengyici Xilou 正乙祠戏楼)
This oldest Peking Opera building from the early Qing-times is in Xuanwu District. Take Subway line 2 to „Hepingmen“, walk south 100m and turn left into the Hutong.
宣武区宣武区西河沿大街220号, 220 Xiheyan Dajie, Xuanwu District (map), +86 10 8315 1649
Beijing Peking Opera Theater (Beijing Jingyuyuan 北京京剧院)
This major venue is out in Fengtai District. Take subway LIne 4 to „Majiapu“ and then a taxi.
丰台区丰台区海户西里30号, No 30 Haihu Xili, Fengtai, Beijing (map), +86 10 6726 7744
Chinese National Peking Opera Company Experimental Theater (Zhongguo Juyuan Shiyan Juchang 中国剧院实验剧场
This venue is in the Xicheng District. Take subway Line 4 to „Pinganli“ and walk 300m in western direction.
西城区平安里西大街22号, 22 Ping’anli Xidajie, Xicheng, Beijing (map), +86 10 5851 9688