Right smack in the middle of the old Marais neighborhood is a grand old museum. I've passed it countless times with barely a glance. The Musee Carnavalet houses the history of Paris in objects, art and 3-d dioramas. The permanent exhibits are totally free, meaning you could sneak in and use the toilets if you need to.
Everything is in French, but you can get an English audio guide.
The building it is housed in is an old mansion, with an additional mansion annexed next door.
Even before Haussmann plowed down neighborhoods to build the grand boulevards, the city is almost always recognizable.
Henri III, aka The king with a pearl earring!
And look, Mount St Michel!
They have a great collection of old store-signs. Most of them were amazingly intricate. Paris wasn't always an educated place, and they came about to let the illiterate know what the deal was.
Kind of unusual:
Spinning is generally a working-class job. Monied ladies did the embroidery, and not usually the flax-spinning.
A really nice surprise is that they had the Mucha-designed Foquett shop. Although he lived in Paris, there isn't too much here that he left behind.
It was glorious.
They had a lot of pretty good local artist representation. Not Louvre-caliber, but some decent lesser works showing daily life and fashions of the city.
It was a nice museum- not overwhelming in any sense. A good way to spend a rainy afternoon, or a day if you are really into your Paris history. I would go back because the pre-history room was closed (they seem to have a rotating list of rooms that are closed on any given day) and they have a lot of early roman artifacts dug up around the town.