f you are Adventurous...
...dare to dive into the Catacombs, a network of underground caves found under the city and used as graves for the cities dead. Interestingly, the section of the catacombs that is accessible to the public is only a small part of the vast network of underground tunnels in Paris and while the official name for the system is "The quarries of Paris" nowadays it is collectively referred to as the Catacombs
If you are Musical...
...Visit the Opera houses scattered about the city. One famous opera house is the Paris Opera house, aka the Garnier Opera house, which features busts of great composers on the exterior.
Odds and Ends...
...Visit the Fragonard Perfume Museum near the Opera Metro Station for a fascinating look into the history and making of perfume.
...Visit Pere Lachaise Cemetery where you can see the graves of such notables as Jim Morrison and Edith Piaf.
There are many activities that one can do in Paris; these are just my suggestions. If none of these interest you, I suggest that you simply explore the city...You can never have a dull moment in the City of Lights.