Hi Francis,
I was waiting for a guest and fell into tourist info mode 😉
The Marais area is great to stay. I have clients there, so I know it very well. It was a Jewish area, then it became more chic and artsy. It has a gay community too, still a lot of great Jewish food, lots of shopping for clothes and second hand fancy clothes and lots and lots of art. You will enjoy walking around there, walking in narrow streets with tiny shops, gallerys and restaurants, to come out in a small place with fountains or art installations and many terraces to rest and refresh yourself. The townhall of Paris is there too. The Picasso museum is there, redone and recently reopened. I did not see it since, but my guests were charmed.
You have an easy walk to the Seine, enjoy walking along the river.
Les Halles was formerly the market of Paris, "the belly of Paris", but that is long gone, outside to Rungis. After a competition, the new center was built, brand new, modern, the big railway station for the RER trains deep down, metro lines in the basement and shops over it. The new parc named after Nelson Mandela is being finished now, last week part of it was already open to the public and immediately taken over by people enjoying to play with the water. It's going to be very pretty, with the view around to several churches. It's full of tourists at night, enjoying to be outside in the terraces. I'd guess that the Marais has an interesting night life too, but as I'm there mostly for business or with teenagers of my family on tours, I don't know that. As Kirsty wants to keep the thread, you will have to put your experiences here as an update 😉