One of the most popular pastimes in China is still the ancient game of mah-jong, which originated in China and is now played all over the world. The traditional game is now played more by older people and in smaller towns and villages, but it is still popular everywhere, and the online version is often played by younger people.
Karaoke is also extremely popular, especially in larger towns, and another public activity almost unique to China is the habit of doing exercises, such as tai chi movements, in parks in large groups. Also outside in parks, hillsides etc, you will often see people flying elaborate kites, often for a festival but also just for fun.
Various card games are played by almost everyone. In particular, any card games that has a gambling element is very much favoured, as gambling remains a preferred activity for many people in China. Otherwise, international sports and other activities are popular in China as they are elsewhere.