You can't get a visa on arrival, please apply for Chinese Tourist Visa(https://www.travelchinaguide.com/embassy/visa/tourist.htm) through China Embassy in Tokyo, you can download the Visa Application Form of People's Republic of China from the website of embassy. The required documents for tourist visa are: your passport, one recent passport photo, one completed visa application form, original and copy of your permanent residence in Japan, sometimes, the hotel reservation and round-trip flight tickets are also required. In addition, you can refer to Shanghai Tours(https://www.travelchinaguide.com/package/shanghai.htm) to plan your 20 days trip in Shanghai.