Forget Versailles! For my money, the better country palace-and-gardens complex to visit when you're in Paris is Chateau Vaux le Vicomte. It's outside Melun, which is a 25-minute train ride from the Gare de Lyon.
Built in the mid-17th century by royal financial advisor Nicolas Fouquet, Vaux le Vicomte was the first project of the Versailles dream team of architect Louis le Vau, decorator Charles Le Brun, and landscape artist André Le Nôtre. Was their first collaboration the more successful one? Hard to say, but I do like how this estate's majesty relies less on sheer glitter and more on the optical wizardry Le Vau and Le Nôtre brought to their organization of chateau and landscape. (There are 81 acres of gardens, 17 of which are water features.) Another compelling factor at Vaux is the comparative lack of tour groups.
I say it's a great day trip...but the Saturday night spectacles in summer (they run until early October) are also worth carving out some time for, especially if you're traveling with kids. The whole place is illuminated by candles, for a flavor of what it might have been like to attend a garden party there when the place was at its peak, and there's a fireworks finale.