上厕所 (shàng cè suǒ) is the most common way to say 'go to the bathroom.' More polite alternatives: 洗手间 (xǐshǒujiān, 'hand-washing room') or 卫生间 (wèishēngjiān, 'hygiene room').
culture
Public restrooms in China have improved dramatically. Major cities now rate public toilets with star systems. In tourist areas, look for signs saying 公共厕所 or simply WC.
