Advance bookings

Only in Paris or elsewhere in France?

And when is your trip?

If you're interested in one of the D-Day tours in Normandy, that needs to be booked in advance. And I believe some of the prehistoric caves down in the Dordogne require advance booking.

Sorry. Only in Paris in June of this year. Spending 2 nights in Chamonix. Will buy tickets to Auguille du midi closer to arrival depending on weather.

The Paris Pass is a tourist hose job -- Never an economical choice. If you want a museum pass, get one of those (the paris pass includes one at higher cost). If you need a transport pass, get one of those -- the one in the Paris Pass, the Visite, is hugely overpriced.

You need to book a timed entry to the Louvre (if you want to go inside it) even if you buy the Paris Museum Pass. I think I read that you must do the same for the Pompidou Centre, but I may be wrong about that. My impression is that the museums' times don't have to be arranged very early, however, it's always possible that some specific times could sell out early; the first time slot is often popular.

The Eiffel Tower needs to be booked early if you want to avoid a potentially long ticket line, but that means you take a chance with the weather.

Check on the Catacombs; I think that's a place that has long lines.

We need someone else to explain the Versailles situation. All I know is that it is mobbed and a lot of folks recommend taking the tour of the private apartments since that part of the palace isn't open to the public except for the tours.

That's all I've got. Others will be able to help more.

felesh, maybe you could give us a list of the sites you’re most interested in?

Since you missed Fontainebleau en route to Paris, maybe go for Versailles? To avoid the endless ticket line, you need to buy tickets in advance on their website. And in June, there are timed tickets available on the hour (highly recommend)
https://billetterie.chateauversailles.fr/passport-with-timed-entry-visite-chateau-css5-chateauversaillesmobile-lgen-pg51-ei552471.html

I've read in several places that people wouldn't recommend Versailles for first time visitors. I do want to see castles though and that's the mothership!

Le Louvre, Orsay, Saint Chapelle, Arc De Triomphe, Eiffel Tower, Catacombs, boat down the Siene.

Other than that, I want to walk and explore. We have 4 full days and a partial day before heading to DLP for 2 nights and home.

Depends on what you’re looking for in a castle. Versailles is more palace, and definitely incredible. But if you’re looking for dungeons and moats closer in to Paris, there’s the Château de Vincennes. http://www.chateau-de-vincennes.fr/en/

Maybe this would fit your schedule better?

Versailles will take most of a day to fully see.

You don't need to book a cruise on the Seine in advance. Vedettes du Pont Neuf is the one I took. They seem to have a boat leaving every 20 minutes. Just head over there and buy a ticket.