What to See
There is no way to answer this completely as there is a never-ending supply of sites, museums, people, shows, and events to suit everyone. It will also depend on your energy level, interests, and how long you are in Paris.
I never recommend that you plan more than one major activity a day, and even no more than four of them in a week's stay.
This sounds kind of skimpy, but hear me out. Unless you are with an official tour, you will want time to be spontaneous. I also still under estimate how much energy it takes to get there, get in, participate, and how tired I am after I finish.
It is also wise, especially if travelling with kids, to have "non-event" days to give everyone down time. If you over-book your stay, you run the risk of getting home and feeling resentful at how exhausted everyone is and how little you remember.
So, do your research and pick your top four activities (per week) that you must see and then choose secondary activities you want to do if you have time. Plan some "off" day things like picnic in the Luxembourg gardens.
So, what are my favorites? When I'm only there for a week, here are my top choices!
1. Musée d'Orsay
This is a fabulous museum full of impressionists and art nouveau. I've read guide books that give you three hours, but short of running through the museum and looking at the art in using peripheral vision, it should take longer. Plan on a full (4 hours) day with a break to enjoy lunch in the restaurant.This museum has sentimental meaning to m because I accompanied a friend to the "exclusive" opening of the museum in 1987. I was also able to see how long the line was from my little window where I lived.
Alternative - if you don't want to spend a whole day but want to see impressionist art, visit the Musée de l'Orangerie, located at the far end of the Jardin des Tuileries.
2. Musée Carnavalet
Located in the Marais, this my favorite history museum as it's all about Paris. I'm enamored with the tableaux of the city when the outer arrondisemtents were still countryside. It also includes artifacts dug up from the Parisi, the first tribe to inhabit the island, Île de la Cité where Notre Dame is located, back in prehistoric times. You can do this museum in about a half-day.
Alternative: Musée National du Moyen-Âge-Thermes de Cluny - This is my other favorite history museum as I love the Middle Ages. This is where you will see the famous "Lady with a Unicorn" tapestry. You can also do this in about a half-day.
3. Tour Montparnass
This is one of my radical choices as it is, for me, a welcome alternative to the crowds of the Eiffel Tower which is usually crowded and slow. The Montparnass Tower is the only sky scraper allowed within the Parisian city limits. It is still the tallest one in Europe and still controversial among the older generation.
It rises above a busy train station, RER and metro station, and a huge center of commerce (mall). The line always seems reasonably fast (like 15 mins!) and you are whisked up a super fast elevator to the observatory floor at the top. It is air conditioned and includes the most amazing 360 degree view of the city and the suburbs beyond. There are little maps all around so you can identify what you're seeing (like that's the Eiffel Tower down there), a dark room with informational movies, a gift store, and a (probably) super expensive cafe. And did I mention it's all air conditioned?!
My favorite part, though, is that you can walk up the final flight of stairs to the roof. I have a great memory of watching a lightning storm, far away over the country side, with my daughter.
Despite my choice for the sky scraper view, you MUST at least walk underneath the Eiffle Tower. It's one of the most amazing sights you'll ever see - and one of my favorites. Warning - eat before hand as the most expensive street vendors, I'm sure, are located here! And of course, the pick-pockets will be busy along here too.
4. Le Chateau de Versailles
This is just an amazing day trip. You do not need to go through every room to enjoy it. Just walking around the grounds can be enough. Unless you are up for a long day, I suggest you choose between the Chateau or the Grand and Petit Trianons and Marie Antoinette's petite hameau. This will be an entire day trip!
What about the other attractions in Paris??
So, you are probably aghast at what I've left out!
But let me say that many of the must-see sites, I consider appropriate for those in-between activities
What other sites/activities to plan
SO MUCH TO SEE, SO