Sure! Here are the sentences filled with the possessive case of the nouns:

1. **Men's** books are on the second floor.
2. Her **parents'** car was not expensive.
3. **Charles's** pen is new. (Alternatively, you can also say **Charles'** depending on the style guide you follow.)
4. This is the **boy's** bike.
5. Those are the **boys'** pencils.