If I want to go to the beach from Changshu, which side of the sea is closest?

The nearest one is of course the East China Sea, if you go south. You can go to Chongming Island in Shanghai. Now the Yangtze River Bridge, the world's largest tunnel, has been completed. You can drive directly to Chongming Island or Jinshan District in Shanghai. The beach tourist area there has just been developed a few years ago.

Go north The closest one is Nantong, but the beaches there are not very good, or you can go further to Lianyungang or Qingdao.

This is relatively far. It is estimated that it takes at least 4 hours from Changshu to Lianyungang. However, it is winter and seaside tourism is off-season, and there is not much fun to go to the seaside now. Now you can consider going to hot springs or other places. If you want to see the sea, you can go to Mount Putuo.