Montmartre is a large hill in Paris's 18th arrondissement. It's home to Basilica of Sacre-Coeur, the big white church that sits at the very top. Here you will find colorful shops, little bakeries with sweet smelling crepes, mulled wine, and painters. I can also guarantee within a few minutes of browsing around, you will be offered a sketch of your own caricature!
If you think of a triangle, consider the base of it to be the section of Blvd. de Clichy and Blvd. de Rochechouart. The tip of the triangle would be the Basilica of Sacre-Coeur. The area between these three points is roughly the area you want to check out! Montmartre is so charming that it will instantly remind you why you booked your flight to Paris in the first place!
*Highly recommend waking up early and spending an hour here having a coffee whilst enjoying the view.