Paris doesn't need an introduction, which is exactly the problem — everyone arrives with a mental image built from movies and Instagram, and the real city takes a few days to push past that and show itself. It's bigger, grittier, and more workable as a solo traveler than the postcard version suggests: an efficient metro system, neighborhoods that each feel like their own small city, and a culture of sitting alone at a café table for an hour that makes solo dining feel normal rather than conspicuous.
Give it at least four days — Paris rewards slowing down far more than it rewards a checklist, and the arrondissement system (the city spirals out from the 1st in the center) makes it easy to pick a few neighborhoods and actually get to know them instead of sprinting between landmarks.