"Marais" means swamp. The evoked swamp was dried up at the beginning of the 13th Century by local monks and the Knights Templar. The word "marais" is also the root of the word "maraîcher" (that you can write with or without the circumflex accent these days, see the hyperlink on"hôtels" here below to know more about that) which means "produce grower" or "market gardener". It does make a lot of sense (for once) because the soil of the former swamp was very fertile because of the numerous sediments which had been brought for centuries by the waters.
After the inauguration of the Place des Vosges in 1612, the Marais became a coveted area to have a house built in for the nobility. Then, later, they moved to Versailles, then, when they came back, they prefered the area of Saint-Germain-des-Prés. In the 19th Century, it was mostly occupied by workers, and promoters later destroyed a lot of the original hôtels in order to replace them with more profitable condos. Fortunately some of the original buildings and streets have survived and this is one of the most pleasant districts to walk around in, with trendy shops and art galleries everywhere (a bit like Saint-Germain-des-Prés but not as superficial and with more social diversity).
Inside the Marais, you also find the Jewish district. Depending on your interest, we can spend more or less time there. We can even have a quick, delicious and reasonably priced kosher lunch there!
During our tour, we can add the inside visit of the Carnavalet museum, the house of Victor Hugo (you'll love it!) or the Picasso museum (but only if we plan it in advance). Depending on your motivation, we can spend between 1,5 hour (orientation tour) to 4 hours in the Marais.
I strongly recommend 4 hours.