If you visit Paris, I would recommend to go on foot, from East to West, along the Seine, and to visit all the main points. First it's Notre Dame Cathedral, will recognize a sure:) after Notre Dame and besids the Seine is Louvre Museum, will not have time to see it all, but at least you see on the outside, see the building, see the pyramid...After the Louvre there is a park take it through it and get to the Place de la Concorde and here begins the Champs Elysees. At the end of the Champs Elysees is the Arc de Triumph. You can climb up to it; I think it's about € 10. Close to the Arc the Triumph there is Eiffel Tower, for sure you will see it. Do not worry as you can get lost, there are subway stations, basically if you are lost, you can go in any direction and you get to a subway station, in the end and also at each subway station of Paris there are maps of Paris and locations of all stations.
Montmartre neighbourhood has a special charm and also the Tuilleries gardens. In one day I saw the Sacre Coeur, the Dali Museum, searched through the towers, seen from outside the Palais de Justice, the Rodin Museum, got into the Eiffel Tower, the Arc de Triumph and walked around the Champs-Elysees. In one day you could see from the outside all the attraction, but if you want to visit them indoors definitely you will need more than3 days, and i mean it :)
By the way, don't miss the Museum of Discovery and antique market on Saturday. The prices are astounding, but worth the walk.
Hotels in the tourist area are expensive, over 90 euros, a 2-star hotel. We stayed in Montmartre with 60 euros per night but were very unwanted experiences... small room, barely found place to put luggage. In the evening we went to the square Pigalle, Moulin Rouge and the Sacre Coeur Cathedral (indeed seen it in the night, its gorgeous:). From the fountain of St. Michael is a Boulevard that you climb: St Michel, on the left are clothes shops, at very good prices, and great clothes also.
The Boulevard Pigalle is not a place for normal people; everything is flooded with sex shops. Kindly recommend seafood: oysters, clams, snails, crabs, and shrimps. Super markets are cheap in the Sacre-Coeur Church district, for example Ed market. Important to know that in the first and last Sunday of the month, there are days with free entrances to museums. So plan your trip in such a way as to catch a Sunday as for example in the evening Gioconada at the Louvre.