We visited Versailles with our children and ended up staying for six hours. The palace itself has some very impressive sights, culminating (for me) in the hall of mirrors. However the gardens are just spectacular. I’d go again just to walk around and marvel at the gardens. We hired a boat on the grand canal which was good fun.
I think the Palace of Versailles is an essential destination for anyone visiting Paris. Great value for money as well with a passport ticket.