I lived there for six months as part of an exchange program in my school. My tips are:
Bring more money than you expect. Food there is delicious, but expensive. If you plan on shopping, double expensive. DO NOT let your Fiance carry his wallet in his back pocket. Front pocket should be fine… but over there I witnessed an instance where tourists were easy prey… they freaked out in a very loud fashion, where the French people just gave them side glances and looked noticably awkward. However on the same note, my first day there two gentleman helped me carry my two big bags off of the metro and pointed me in the direction of my appartment complex. So good, and bad. 🙂
Know a tiny bit of the language. If you don’t know some French already, just try to learn some of the basic phrases that you’ll need. Most people in Paris do know English, but attempt French first. They will appreciate the attempt (some of the French are very sensitive to the dominance of the English language), and will be much friendlier and more helpful.
Paris is a large, but well planned city. The truth is, unless you are staying 2+ weeks you will not be able to see every single thing. I certainly didn’t in 6 months! So research and find out what is most important to you.
While the Eiffel Tower is iconic, in my opinion the very best view of the city is from the Arc de Triomphe, and it is a much shorter wait. From the Arc you can see how Haussman (dude that came in and replanned the city) redesigned the city. It is very cool!
Places I loved:
Montmartre, favorite part of the city hands down. So many cool cafes and art work. There is often random musicians playing around this part of the city. There is also the love wall, that is in that part of the city that you might want to check out (photo below). Also the Sacre Coeur is a great place to take a baguette, listen to some free music and just hang out. Also a very cool view of the city.
I also really loved the museums. I am a musem freak, though, and you’ll have to decide what you like. The Louvre is huge. You really can’t do it justice in a day, but the Musee d’Orsay is so, so fabulous. I love that it is in an old trainstation, too. There are other ones, Picasso, Rodin, etc.
The Garden of Tuileries is popular, and the park is lovely. So is the Bois de Boulogne in the day time.
If you like cemetaries, Pere La Chaise is worth the visit, and is not crowded at all. It is away from the city center though, and many people opt out of this one.
If you like modern sculpture, you should go to La Defense. It is the business district, and there are so many fountains and modern sculptures, and La Grande Arche!
Also, the Latin Quarter, near le Centre Pompidou and nearish to Notre Dame and the riverbanks is great for eating, and just relaxing. The riverbanks are also fabulous.
Also if you take a day trip out to Versailles, it is lovely, and worth it.
There is so much I’m leaving out, but this is already so long. I’d just do some research and find what you want to see… or just chill and people watch. Not sure what style you are going for.