While Paris is wonderful, if you want to spend a day in the countryside you can consider either Champagne or the Loire Valley. From Montparnasse to Tours it's 55 mins via TGV, so you can be ready to explore one or two chateaus in a day and return in the evening.
There are buses to take you to the chateaus from Tours Gare and cost around 49 euros per person for a full-day tour- you'll visit Chenonceau, Amboise then Chambord which is quite a lot in one day.
While I would recommend a full day for each of these attractions, if you haven't the time it's a great day for the whole family and no need to drive or hire a car. Lunch is usually at Amboise which has lovely cafes, nothing really really gastronomic but good for lunch.
You can probably organise tickets for the chateaux pick-up via SNCF (gare) or www.tourevasion.com for more Loire Valley activities
Paris-Tours 55 mins via TGV
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