I was just in Paris in September. My recommendations for a super-short 2.5-day trip, in order, would be:
-
Catacombs (buy tickets ahead)
- Take a boat cruise on the Seine - if you don't have time to go to a lot of things, this will give you a great view of the city
-
Sainte Chapelle
- Eiffel Tower - visible form the boat cruise. I personally wouldn't go up it
- Wander through Le Marais and Montmartre
- Take a TootBus hop-on-hop-off or similar that goes by Notre Dame, Arc de Triomphe, some of the more landlocked areas
- Eat all the crepes, croissants, cheese, and chocolate
There are so many amazing museums. But it's really subjective - not everyone loves art, and not everyone loves all kinds of art. I thought d'Orsay and L'Orangerie were more enjoyable than the Louvre, just because I really like impressionism, surrealism, pointillism, and Cubism. I wish I'd had time to go to the Picasso museum and out to Givenchy to Monet's water lilies bridge. But with 2.5 days and museums taking a while, you'll really want to make sure you research what is at each, select accordingly, and then budget time.
Similarly, I loved Versailles, but it's going to take pretty well a whole day to travel there, tour, and travel back, so you'll want to prioritize accordingly.
My biggest recommendation is to use something like Google Maps or Sygic.com to plot out your prioritized items, group events based off of geographical proximity, and budget accordingly. Keep in mind that Paris sort of shuts down on Sunday, so always check what is open which days.