There’s obviously sooo much to do. Might let us know what you’re into, to narrow it down some…
I loved the Musee D’Orsay. If you like the modern art, do Centre Georges Pompidou as well. See famous people’s graves (sadly, the only one I can recall is Jim Morrison) in the beautiful pere lachaise cemetary. Endless boutique shopping in St Germaine… Monet’s water lilies get a gallery of their own (across the street from the Louvre, I think)...