Yeah fair point, I think what I'll do is just pick a few things like eiffel and just walk around :-). Gives us an excuse to visit again
My first trip to Paris was 23 hours. Arrived via Eurostar at something like 5 pm, and departed at 4 pm the next day. That probably doesn't count as a "day trip" per se, but depending on train times and your intentions, you could probably get a good taste/feel for the city in that timeframe. For the evening we arrived, I had booked a dinner cruise along the Seine and it was lovely, particularly when we cruised past the Eiffel tower all lit up. The next day I foraged for breakfast at a nearby butchers and bakery, and ended up with some pretty incredible pate, cheese, baguette, and croissants. We then managed to do a decent amount of wandering around the city, but had our own particularly entertaining impromptu and lengthy photo shoot in the grounds of the Louvre.
I've been back since, but think I actually prefer a short sweet visit like this. Less time for the French-ness of the people in France to start to get to me.
If it helps, this is the dinner cruise I booked:
Calife (note: I think I remember having to do some sort of confirmation of payment over the phone in French, but I worked with three Frenchies at the time so just got one of them to translate)
Also - I wasn't sure if it's completely clear to you from the previous posts: he will need to bring
both his Passport (to enter other countries in Europe) and his BRP (to re-enter the UK).