If you are going to be in Paris for just one day, there's no time to waste !
If you arrive by plane at Paris Charles de Gaulle airport, get a RER ticket to take you straight to the centre of the city. Buy two for the return. They are the same ticket.
Take the RER B2 or B4 direction of Robinson or St Remy les Chevreuse and alight at St Michel notre Dame. Take the RER C5 or C7 direction of Versailles Rive Gauche or St Quentin en Yvelines and alight at Champs de Mars/Tour Eiffel.
If you arrive by train at any station, get hold of a book of 10 single tickets which you can share with anyone else in your party. If you have an left, don't worry, they do not have an expiry date. Depending on which station you arrive at, make your way on the RER either to Champs de Mars/Tour Eiffel or by metro to Bir-Hakeim .
There you can have a look at the Eiffel Tower to your heart's content and take pictures.
Cross the road for ...
A CRUISE ON THE RIVER SEINE
THE HOP ON HOP OFF OPEN TOUR
You'll find a stop on the same pavement as the Eiffel Tower. And for the
rest of the time you have, do a sightseeing tour of Paris with
commentaries and hop-off to rejoin the public transport when you have to leave.