Try to join a tour at 1933 Old Millfun. But if not, this historic structure is still worthy of a photo from the outside.
You're spoiled for sightseeing choice in Hongkou District. 1933 Old Millfun is a good place to start here, but there are lots of other interesting spots to visit. The Bund, Jing'an Temple, and Nanjing Road Shopping District are close to the historic landmark and will fill in your day nicely.