I didn't know about Chapelle Expiatoire until my wife mentioned it plays a part in the Revolution. The Chapelle Expiatoire is a quiet, understated, memorial to the many who were guillotined at the Place De La Concorde.
The gardens around the memorial are free and are used by workers at lunchtime. There are seats and shade if you need a rest.
If you are interested in the French Revolution or you just want a break from the madness, this is a good spot to rest.