Please let me know the things I can do during a visit to Beijing

Well, you don't have enough time to tour around many scenic spots in this city.
But you can enjoy the nightlife. Going to Wangfujing Avenue and taste delicious food. And there are also many stores that sell interesting souvenirs. The costs of the food and souvenir are affordable. So don't worry.
In early morning, you can also go to Tiananmen Square to appreciate the flag-raising ceremony, and Temple of Heaven or some nearby parks to watch local people do Taichi and other morning exercises.