I was in Paris last July for about a week, and here's my advice:
1. Get a guidebook and plan out what you most want to see. I'm afraid my travel group did not do this very well, and while we saw most of what we wanted to, we didn't get to see everything because we didn't plan this out very well. And find out what your travel partner will enjoy and try to be fair about how you spend your time. My husband and I could have spent 5-6 hours in each museum, whereas my mother and sister were ready to leave after 1-2 hours. If you can plan out your visits to museums or other sights in advance (like which galleries are 'must sees' for you), you can save a lot of time. If you plan to go to a lot of museums/sights, the Museum Pass might save you some Euros and lets you skip lines at some of the busier locations:
http://en.parismuseumpass.com/2. Rent an apartment so you can cook, it seriously saved us so much money--I'd suggest going out for lunch when you're out and about walking around and sightseeing and to give your legs a rest, but have breakfast and dinner at your home base--find out where the nearest grocery is to your apartment, and plan simple meals like omelettes and pasta
3. Take time to just enjoy being in Paris. Bring a book or a sketch pad or whatever you enjoy doing during your leisure time, and just sit in a garden or park and watch the people go by. Some of my favorite memories of the trip are sitting the shadow of the Eiffel Tower, playing a card game with my sister and husband, or sitting on bench near the Sainte-Chapelle, eating rice pudding and drinking Citron Presse (so good, American lemonade is nothing like it) from the Monoprix around the corner from our apartment, just talking. Walk along the Seine, listen to street musicians, buy a baguette or package of macarons and munch as you walk, etc.
4. Brush up on a few French phrases like excuse me/sorry, "where is the...?" and so forth. I thought I had a pretty good command of French, but not surprisingly, actual French people speak the language much more quickly and I was soon overwhelmed. Learn how to order gelato/coffee/etc. in French, if possible.
Recommendations:
1) Amorino gelato (there are a few shops throughout Paris) - try the flavor that looks like lemon (can't recall the name right now), it's even better than lemon and especially good mixed with raspberry (framboise)
2) the Musee d'Histoire de la Medecine was really weird but my sister wanted to go and it was kind of awesome; it's just a room at a university with lots of weird old medical implements and stories of discovery of illnesses and treatments that are wonderfully horrifying -
http://www.univ-paris5.fr/CULTURE/Musee-d-Histoire-de-la-Medecine - limited hours, and spring for the brochure in English if your medical French is rusty ;)
3) the Musee d'Orsay was my favorite art museum of the trip, quickly followed up by the Musee de l'Orangerie which houses Monet's water lilies - we did not make it to the Louvre, but I don't feel that I missed out since the Orsay and Orangerie were so excellent
4) if you like stained glass, Sainte-Chapelle can't be beat
5) I know people say going to the top of the Eiffel Tower isn't worth it, but we went late in the evening, and it was lit up and beautiful, and even though it was crowded, and we missed the Metro cut off so had to taxi it back to our apartment (Metro stops at 1:00am on the dot!), I think it was worth it--we actually saw a guy propose to his girlfriend at the top!
6) Why are people saying to buy bread and
butter in Paris? Buy bread and
CHEESE! :D
