The Musee du Louvre is usually on everyone's "to do" list. Originally constructed as a fortress in 1190, it was the royal residence from 1500s until the French Revolution. Reserve an entrance time slot online to avoid the long lines. I spent over 4 hours here and still barely scratched the surface. Pick 2-3 areas of focus and a few signature pieces in each. I chose European masterpieces (Mona Lisa, Wedding Feast at Cana and Coronation of Napoleon), sculptures (Venus de Milo, Mercury and Winged Victory), crown jewels, and Egyptian antiquities. I didn't see everything but enjoyed what I did see. Go as early as you can (I had a 9:15 slot) to avoid the crowds and head immediately to Mona Lisa (there were less than 50 people in line).