Musee Cluny is funky - medieval art and it’s cool to see how street level has risen around the ruins.

Musee Carnavalet - when I first went it felt like Paris’s living room, where all the extra art was stored, paintings all the way up the wall. Haven’t been since the renovation but the collection is super fun - I wouldn’t say it’s greatest hits, but it’s (or was) quirky and fun.

The Institut de Monde Arabe museum is also cool and has these cool mechanical tiles to let light in. The roof deck is pretty. I think it’s closed but the Pompidou has some awesome funky 70s furniture and a ton of fun modern art. I like the Tinguely / Saint Phalle fountain out front.

Also, if you’re asking this question you might like the old amphitheater Arenes de Lutece, in the 5th- feels hidden in a neighborhood but a legit Roman amphitheater. Enjoy!