Pere LaChaisse cemetary is absolutely superb - so beautiful, and fascinating, not at all macabre. It is WAY out of town though.
If I only had one day then I would just mooch around the streets absorbing the atmosphere - picnic lunch by the Seine at Notre Dame cathedral (the local shopkeeper gave us free plastic glasses for our vin rouge); and/or wander around the Louvre and the Tuilleries gardens next door.

Things to avoid: the crowds at the Sacre Coeur (and the steep climb); the African hawkers at the Eiffel Tower.

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~ What do I think of Western civilisation? I think it would be a very good idea ~ Gandhi