Probably Musee d’Orsay in Paris. It holds many of the most famous paintings ever. You can walk right up to each piece and get a close look. And it has several nice cafés where you can sit and have lunch or a coffee. It’s very chill.
By comparison, the Louvre is a mad house, the popular stuff is roped off, and the cafés are more like a snack bar.
If you’re into U.S. (pop) culture, I think it’s hard to beat the Smithsonian in Washington D.C. It’s got historic aircraft, movie props, costumes, etc. Fun stuff. And it’s mostly/all? free so you can spend the day going in and out, having lunch nearby in DC, seeing famous monuments right outside, etc.
Another one in the US is The Getty in LA. Absolutely gorgeous inside and out and also has an appearance in a ton of media, including the final shootout in GTA V. It was really surreal getting to walk through the place having seen it so many times before.
Musée Robert Tatin. Robert Tatin created an Univers of art naïf around his tiny house that become later a museum.
It’s a mix of sculpture and painting, outside and inside. People will find it weird and strange. I found it amazing and representative of a society.
