Versailles might not be the best town to party. However it is fairly easy to mix with the locals.
There is a little concentration of bars and restaurants on the Place du Marché, at the junction of the Rue de la Paroisse, and Rue du Maréchal Foch. This is where most young people go out at night. Tables outside the terraces are plentiful in the nice months. Exploring the walking alleys from the Place du Marché can also reveal less know restaurants.
Another popular bar with Versailles' youth is the O'Paris, on the Avenue de Saint-Cloud, very close to the Château's Place d'Armes on the right-hand side when facing the Château. They often play sport events inside. Tables outside are also available when it is warm enough.