hi - if there's one thing Paris does not lack, its cafes. You can find them almost everywhere, but best places for a coffee and croissant are in Montmartre and Opera districts.
this might be of interest also:
http://www.guardian.co.uk/travel/2009/apr/15/paris-pavement-cafes-bars-france almost 10 years ago
Aside from the obvious sites eg. Eiffel Tower, Louvre etc I really love Sacre Coeur & the gardens in front and also Jardin des Tuileries (Gardens just near the Louvre)
Particularly in Summer these are great spots to soak up some atmosphere and are close to sights that are usually on the 'must see' list.
Slightly off the beaten track, I found the catacombs very interesting. They're a little harder to find and aren't very well advertised, but certainly interesting and it gives some history as well. almost 10 years ago
Paris is a wonderful city for just walking around and catching the atmosphere. On every street there is a bar or cafe that you can sit at and watch the world go by. Down by the Seine there are gardens and seating available.
I picked a different area each day and just walked/rode the bus or metro until I felt I had done and then moved on. almost 10 years ago