fleshing out 25th anniversary trip September-ish 2027

Paris in a day boggles my mind. But I love to spend a week in a location and get a feel for it. For one week, do not attempt more than two cities please, not the four countries you offer. I hear you on the daytime flights to LHR. From there,you could fly to Paris or Brussels or Amsterdam.

Congratulations on your 25th anniversary. Here are some thoughts from me:

-Buckingham Palace tours this year only go through 27 September, so keep this in mind when you are booking.
-I would strongly disagree about seeing all of Paris in one day! But I suppose it depends on what you actually want to see. You would need a whole other day to visit Versailles and multiple more days to visit Normandy.
-With one week I think London and Paris is plenty, just split the time there- or just stay in one or the other and visit the surrounding area (e.g. Paris and Normandy).
-Is there a specific concern your wife has with the channel tunnel? Is it being in a tunnel? The tunnel part is very short, it's only about half an hour of the 2.5 hour journey if I remember correctly. You can also fly, but it takes so much time to get to and from the airport on either end you will waste a lot of time that way. There's also the ferry but you would need to research which routes take foot passengers, and then of course there is the time spent getting to and from the port at either end.

Good luck with your planning!

I really enjoyed my Rick Steves' tour of Paris in a week, but looking at your list, he doesn't have one tour that would accomplish all of your must sees. Maybe the South England one? I don't know.

Happy anniversary!

I really enjoyed Paris in 11 nights, then back again for 3 nights after finishing a Normandy trip last fall (both D Day and Medieval history) but to each his own. I'll be back for 4 nights soon (Paris, that is).

Happy anniversary!
About 5 or 6 years ago we flew to London, did ou sightseeing, and took the chunnel to Paris and spent about 5 days there, I don't recall exactly. I, too, disliked the idea of the being enclosed in a vehicle under the sea, but it was a breeze! It was easy and we were on the continent in no time. I recommend it, and as I said, I'm a bit claustrophobic.

We've been to Paris 4 times now, and I still have a list of things we missed. One day, is certainly not enough, and two would be better, but if you have the option, even more is better. FWIW, my first time in Paris, we had lunch at the Eiffel Tower, and I enjoyed it, but it was my first time there, and it was more than 20 years ago. Re the Louvre, we did a tour once, and it was very helpful, because the guide took us directly to the "most important" or most famous works, and it was a time saver. The Louvre is huge!
If you have specific works you want to see, you can pinpoint them using a link on the Louvre website, which also saves time.

Enjoy!

My brother has been and tells me that in his opinion, it can be seen in a day

Wow.

If Buckingham Palace is an absolute must for your wife, as is a daytime flight from the US to LHR, then I would suggest you tour the palace, and then head to Paris for the remainder of the trip. There are many options for direct flights from London to CDG. This link may help to explain your ferry options if she does not want to travel by Eurostar, and if you decide against a flight: https://www.seat61.com/trains-and-routes/london-to-paris-by-ferry.htm Consider the fact, that you will lose a large chunk of one day by traveling from London to Paris. When planning a trip, think about it in "nights" not "days".

In my wildest dreams, I can not imagine seeing Paris in just one or two days. You mentioned Versailles, and that will take you a half-day at the very least. After spending a few days in Paris, you could travel by train from the Saint Lazare station to tour the beaches of Normandy. IMO there is far too much to see and do there for it to be a day trip. Would you be open to renting a car at a rental company located next to the train station in Caen? After spending time touring the beaches, you can take the train back to Paris for your flight home.

I love the Netherlands, and thoroughly enjoyed it. Bruges is small, so you would not need a lot of time there. Amsterdam can keep you busy for multiple days. I was not a huge fan of Brussels. With that said, I would recommend you limit this trip to London and France, since you do not have a lot of time.

Thank you all. This is stuff we could've PROBABLY sussed out eventually, but what a head start. As for the "Paris in a day" it sounds like that's off the table, haha! I can't remember the exact context of the conversation, so I probably should've left that out.