Long Lines

You'll find ques shorter now than a month ago. But the larger, more popular museums are busy all year. Some need prior ticketing and some are fine to walk in. The trick is to go early or late and on the slow days of the week.

There are definitely sights for which you should buy tickets in advance. You need to tell us where you're going so we can tell you what they are.

Louvre
Orsay
Paris Catacombs
Saint Chappelle

These are some of the locations the kids picked

Have you checked the Paris Museum Pass ? Depending on how many covered museums you want to see and the time you have to see them, the pass can be a time- and money-saver. Often a specific entrance for pass-holders (everyone goes through security check.)

You maybe already know this but, the owner of this forum writes books that extensively cover strategies to NOT wait in lines.

At the Louvre, I had great luck arriving a little more than 30 minutes before opening with no bag and museum pass in hand. I have the photo of just me and Mona Lisa to prove it.

The catacombs have had a long line every time I've gone past, that's why I've never been through it. I think the idea is to book a timed entry but I'm not good with reservations.

The Orsay was horrible on a free Sunday. Other than that it's not too bad with a pass.

I went to Saint Chappelle for an early evening concert. It's beautiful and the lines aren't bad at all. I've also been on a free day, we arrived at least 40 minutes early. We weren't first in line but it was manageable. The line behind us at opening was likely at least an hour.