I visited Père-Lachaise with Michel Dansel, author of Au Père-Lachaise: Son Histoire, Ses Secrets, Ses Promenades, and it was a great treat. It won't be the same with just the book, but touring the cemetery with a copy in hand should still be grand.
I particularly loved the Musée Rodin, not just for the art but for the building and the grounds.
The best tour I went on in Paris followed the remnants of the wall Philippe-Auguste built around the city during his reign. The tour involved stops in people's apartments in buildings that had been erected around the wall. If anyone is still conducting that tour, take it.
It's fun to visit every one of the Métro stations with Art Nouveau entryways. Your quest will take you all over the place, and you'll see lot of neat stuff on the way.
In general, just walk and walk and walk. And if you happen to end up on the rue de Fleurus (which you may, because it's one of the roads that takes you to the Jardin du Luxembourg), say hello to no. 43 for me; that's where I lived.
Most important, eat and drink a great deal.