* Wander the streets aimlessly,
* Sit at a cafe outside people watching,
* Lots of history museums, even ones off the beaten path that no tourists go to are quite worth the visit.
* Enjoy omelets that are 50% butter, almond croissants, Nutella and banana crepes for next to nothing in money. Then blow your day's allotment of cash on a great dinner.
* Try good French wines. You won't care about finding activities afterwards.
* Sit in a great restaurant where the tables are so packed that you have to pull a table out to sit down. Then spend the evening talking with those at nearby tables. Here is a restaurant like that:
http://www.tripadvisor.com/Restaurant_Review-g187147-d719212-Reviews-Le_Petit_Pontoise-Paris_Ile_de_France.html
* Do a few of the big sights: Notre Dame, Louvre (but take the metro in directly to the basement to avoid the 1 to 2 hour line the tourist suckers suffer through), Versielle, Eiffel tower at night, etc.
* River walks / cruise?
* Paris Opera House (the setting for the Phantom of the Opera). Go on a tour, watch ballet, or even an opera. https://en.wikipedia.org/wiki/Palais_Garnier
* The sewer tour was okay. It is more a history of Paris museum than anything else.