How to best see Paris on short time line?

You might try one of the hop-on-hop-off bus systems, to get you from place to place, give you time to stop where you want, and see it all from aboveground. L'Open Tour seems to be favored by many on this board. I haven't done this in Paris because my visits have been longer, but I have in other cities.

I'd also suggest looking at Rick Steves' Pocket Paris or a similar guidebook. These books recommend sights for visits of various lengths, and provide enough info to help you decide for yourselves.

Ile St-Louis is a beautiful and centrally located place to base yourselves. Don't try to "hit" all the sights in such a short trip, assume that you'll return for a longer visit and enjoy to the fullest what you can now. Pace yourselves, spend some time strolling and café-ing and people-watching.

Hop-on-hop-off bus systems will give you a scenic above ground view, but you might waste a good bit of time getting from place to place. The Metro system is much faster. The suggestion about the Rick Steves' Pocket Paris guidebook is great because of the amount of information it will give you about the many options available to you and then you can decide what strikes your fancy. There simply isn't any way to see all the major stuff in the time you have available. We recently stayed on Ile St-Louis so send us a PM if you need specific info.

I agree with using the metro and once familiar you can save a lot of time. Also a good help is parisbytrain.com, so you can do some work at home already. Many major tourist sites are within reasonable walking distance from metro line 1 like:

Bastille
Place des Vosges
Notre-Dame
Centre Pompidou
Louvre and even Musée d’Orsay
Place Vendome
Place de la Concorde
Champs-Élysées
Arc de Triomphe
La Défense

From Arc de Triomphe line 6 runs to Trocadéro for going to the Eiffel Tour and for instance in opposite direction line 2 to Anvers for a visit to Montmartre. I guess the closest metro station of line 1 from Île Saint-Louis is Saint-Paul, but you can use the more closer station Pont Marie or Sully Morland of line 7 that crosses line 1 at Châtelet.

What day (and time) do you arrive/depart ?

Day 1
St Chapelle
Notre Dame
(Luxembourg Gardens ??)
Musee d'Orsay

Day 2
Eiffel Tower
Place des Vosges & Pompidou Centre
(Tuileries/Orangerie ??)
Louvre

Half Day (if possible)
Sacre Coeur+Montmartre
Arc de Triomphe+Champs Elysees

You are the best ones to decide what are "must sees". If you have always wanted to visit the Louvre, then it would be must see, but if you are indifferent to the type of art at the Louvre (and dislike crowds) then skip it. If you like modern art or the Impressionists, then go to the Pompidou or D'Orsay. 2 1/2 days is a short time for Paris so be selective and assume you will return. A good guidebook such as Rick Steves' will show you when museums have late hours, which will help maximize your sightseeing. Do leave time for strolling a couple of neighborhoods and cafe sitting. Bon voyage.

Hi, catch one of the evening cruise boats down the river to see a lot of sites that are even more beautiful in the evening light.

I would second Jean's suggestion of a Seine cruise, and recommend taking Vedettes du Pont Neuf after your Louvre visit to rest your feet for an hour. http://vedettesdupontneuf.com/home/

I agree that the boat tours are awesome. Vedettes de Pont Neuf was great, but doesn't offer dinner. You can find dinner cruises, but they can be pricey. Conciergerie is very close to Notre Dame if you want to check that out. Get your tickets to the Louvre in advance, if you can, to avoid a very long line. Otherwise get the tickets from a tobacco shop at the bottom of the mall at Carousel du Louvre (only takes about 30 minutes).

I'd take Vedettes du Pont Neuf and then have dinner somewhere on the Left Bank or the Marais (both are walking distance).

Go to an evening concert at Sainte Chapelle, the performance that starts before sunset and ends after it. Get there as early as possible after doors open and you'll have time to walk around the chapel before the concert starts to view the stained glass and take photos. If you take binoculars you'll have great views of the windows - during the concert too.

Arc de Triomphe - around sunset is the best time, watch as the sun sets behind La Defense and the lights go on around the city and on the Eiffel Tower. If you can, visit the Louvre in the evening - Wed or Fri. Consider starting with a Fat Tire bike tour to get a good overview of the city without getting stuck in traffic on the HOHO bus. Paris is beautiful after dark. The buildings, bridges, and fountains are illuminated until around midnight. The city can be magical early in the morning when it's quiet. Sleep when you die.