One of the most popular day-trips from Paris is a visit to the the Palace of Versailles. The palace itself is a huge draw with the royal and state apartments plus the famous hall of mirrors among some of it most popular features. Add to this the astonishingly over-the-top formal gardens (saa below) and park plus the estate of Marie Antoinette which include the Grand Trianon and the Petit Trianon and you have the makings of a great day out.
Official website: https://en.chateauversailles.fr/
Visiting the Palace of Versailles
In the past most visitors to the palace turned up with tickets in hand which made for long queues at the security line outside but now timed admission tickets are compulsory to see the main palace. These tickets guarantee admission with maximum half-an-hour wait under normal conditions.
For opening hours and ticket prices visit the palace's official website