The tests would be strengthened if we also tested that the original rental items returned to their original state. Do this by generating another table at the end of the test to ensure this.
2.
When a return is picked from the list of returns, all the rental items in that rental are returned. Change the test generator to make partial returns.
3.
Rather than storing the setup data in the program, supply this data through Fit tables, with a special fixture to run the test generator.