I'd echo that Sacre Coeur is great in the evening. The best free spot in Paris, take some food, sit on the steps. A guy with a bucket of ice will probably walk up and sell you a bottle of cold beer...
There are is often someone doing a side show or similar.
Pompidou Centre used to be pretty good too.
If I remember correctly (and I cannot remember the details fully - old age!) take the metro to La Defense. Not much there, but good for a few photos, but there is a big 'straight' on the metro line and you do reach 'warp speed' (excellent

).
Also, do the Lourve, but not really for the Mona lisa, as mentioned earlier it's a let down. The rest of the museum is pretty good though.
Another good 'free time' - walk from Arc de triumph, Champs Elyees, etc.
Buy some bread and wine, just watch the world pass by. :wink: