The big one
The Musée d'Arts runs from Italian primitives to the present — Georges de La Tour, Ingres, Courbet, Monet, Kandinsky — in a nineteenth-century palace joined to a translucent white extension.
It shuts on Tuesday, which catches people who assumed Monday.
The specific ones
The Jules Verne museum is small, on the hill above the quays, and about the making of the books rather than the man. The château's museum is the history of the city and does not flinch from the slave trade.
Le Lieu Unique is not a museum — it is the LU factory as an arts centre, and you can walk in for nothing.
Closing days
They do not all shut on the same day. Some close Monday, some Tuesday. Check the specific one before building a day around it — this is the single most common wasted morning here.
Musée d'Arts de Nantes
Musée Jules Verne
Château des Ducs de Bretagne
Le Lieu Unique