take the subway to ile de la cite, see notre dame, walk to the louvre, go across the river to the orsay, walk back across the river, through the tuileries garden and up the champs elysees to the arc de triomphe (be sure to take a good at la defense from there), and then walk over to trocadero and then down the hill to the eiffel tower and the champs de mars
that'd take the better part of a day depending on how much time you spend in the museums.
depending on when you get into the city you could have lunch at a cafe on boulevard saint germain in the left bank.