4 days in Paris - seeking advice

Are you asking about tickets for a Seine Cruise? This is a decision you can make at the last minute as you can generally walk up to any of the regular boat lines. The dinner ones need reserving but I don't think you want to do that.

Buy all your tickets, - Eiffel Tower, museums, etc directly from the official websites. There are quite a few shady sites from 3rd parties so make sure you are not in a hurry or distracted and book only on the official website. On your other thread about the Louvre the website is listed in one of my posts.

...your thoughts on a cruise down the Seine?

We've done several night cruises with Vedettes du Pont Neuf and have always been pleased. The live commentary was very good. We have always been able to just walk up and get tickets, but you might want to check just to be sure since June will be somewhat crowded. You will want to take one that starts as darkness arrives so you can see the Eiffel Tower sparkle.

Some people opt for a dinner cruise, but, as Pam indicated, You probably don't want to do that. Quite expensive, so so food, and it's hard to enjoy dinner and sight see at the same time.

Look here for water cruises.
I did one in October and really enjoyed it…the one starting in Canal St Martin and going out along the Seine.

https://www.canauxrama.com/en/?gad_source=1&gad_campaignid=21312357062&gclid=Cj0KCQiAm9fLBhCQARIsAJoNOcvxC5E1mH5ad9QSHrb8qLzltknpiaEhjMh6ITlFHx9W4NXwJ9sJV-4aArI0EALw_wcB

Thank you all. I will purchase tix directly from the website and do the cruise at night. Any other must do ( I realize there is a lot) I would love to hear. Thanks again, Lisa

There are several companies that offer cruises on the Seine. I prefer Vedettes du Pont Neuf. They use smaller boats and have live commentary in English, French and sometimes Spanish.

You'll want to get in the boarding line early so you can get good seats upstairs. The nicer the weather, the busier all the cruise companies will be.

Be advised that your 18 year old is an adult in France. Expect to be charged accordingly.

I see no need to pay for a private company Seine cruise. IMO the Vedettes de Pont Neuf are not noisy. We always sit up top and people are listening to the narrator. I never felt crowded there either. Perhaps Bateaux Moiuches are more crowded. I have noticed some boats that seem to be very crowded, however. Vedettes are a good choice, about 17 euro pp, can be ordered online ahead of time for a specific time, if you wish.

Another vote for Vedettes du Pont Neuf. Reasonably priced. Basic but good. I purchased directly from their ticket booth..

I also recommend the regular one hour cruise (not the "sunset cruise") with Vedettes du Pont Neuf. Note if you want to do a nighttime cruise that is timed for the Eiffel tower sparkles, you have to go on one that is after sunset (the tower doesn't do the sparkles until then). In early June that will be a cruise that is 9:45 p.m. or later. You can buy tickets ahead of time but I recommend arriving at least 30-45 minutes ahead of time to get good seats up top of the boat.

Do you have four full days or are some of those days arrival/departure days? If the Louvre and Eiffel Tower are priorities then be sure to get tickets ahead (and as noted, use the official websites). You don't say what days of the week you're there, but the Louvre is closed Tuesday. As for other "must do" things, you may need to provide more information about your family's interests to get helpful responses. With kids that age, a bike tour might be a good activity. I did one with Bike About Tours that was a nice way to see the city.