Batobus ticket up and down the Seine. Get off at the Louvre for the Tuilleries and to see the Eiffel Tower. Wander round Notre Dame and the Islands. Go to Montmartre to see the Sacre Coeur and the lovely squares and artists on the hill behind Place des Vosges and the Marais - lovely old arcaded buildings and some very nice foodie type shops near there. Lots going on near Les Halles. Lots of cafes/restuarants there for cheap eating. Ditto Left Bank near St Germain.