See the Louvre and Versailles for less than booking both separately.
What you get at the Louvre Museum: Timed entry within your reserved slot, plus access to the permanent and temporary collections. Explore at your own pace once inside.
What you get at the Palace of Versailles: Timed palace entry with access to the gardens, the Trianon Estate, and sites like the Gallery of Coaches and Royal Tennis Court.
Why this combo works: The Louvre gives you the key artworks most first-timers come to Paris for, and Versailles covers the royal palace-and-gardens side of the story. Since both entries are timed, splitting them across two days keeps your slots easier to plan.