I'd buy a metro pass for a few days, its ridiculously efficient. The bus takes in the sights, but I preferred to go via the metro as the station entrances themselves were sometimes a work of art. As for places to go, I'd definitely recommend the Gardens of Luxembourg and Sacre Couer although keep an eye on your wallet at the latter as I saw someones iPad being half-inched last time I was there. The Perl Lachaise cemetery has the graves of Edith Piaf, Jim Morrison, Oscar Wilde and Moilere and although it fascinating me, my missus thought it was a bit macabre.
The usual Eiffel Tower, Notre Dame, Louvre are worth a visit, but usually heaving. Everyone has a different experience as its so huge, but its worth doing as much as you can.
Disneyland Paris is par for the course, very expensive for scran but the rides are pretty cool. Head underneath the castle to see the dragon chained up.