@olynch Random selection of personal favorites: parc Montsouris (https://en.wikipedia.org/wiki/Parc_Montsouris), bois de Vincennes (https://en.wikipedia.org/wiki/Bois_de_Vincennes), basilique du Sacré-Cœur (https://en.wikipedia.org/wiki/Sacr%C3%A9-C%C5%93ur,_Paris), hôtel des invalides (https://en.wikipedia.org/wiki/Les_Invalides). (I would have recommended the palais de la découverte if it wasn't closed currently.) The catacombs are very interesting but you need to book a week in advance. Also, @gro-tsen.bsky.social wrote blog posts in French about parks, forests and hikes in the region:
David Madore's WebLog: Guide du tourisme en Île-de-France : partie 1 (généralités et petite couronne)
David Alexander Madore's WebLog / Diary
(www.madore.org)
David Madore's WebLog: Guide du tourisme en Île-de-France : partie 2 (le sud et sud-ouest)
David Alexander Madore's WebLog / Diary
(www.madore.org)