If you only have a few days, prioritize all the most known spots!
Notre Dame has recently reopened and is worth the visit. We personally recommend the Musee d'Orsay over the Louvre (but don't miss visiting the Tuileries and taking a photo by the pyramids in front of the Louvre). For either, think about booking in advance to skip the line. Don't miss climbing to Sacre Coeur and wandering Abbesses and Montmartre for cute shops and a beautiful view of the city. A Seine cruise is a great way to see the sights - Bateaux Mouche is easy to walk straight onto.
We would recommend only going to Versailles if you have more than a few days in Paris.
If you have seen the major sights, you could wander the cute streets around the Canal St Martin, learn about the history of Paris at the (free) Carnavelt Museum, enjoy a walk through Buttes Chaumont, or have tea time at the Grand Mosquee!