Hi CLKJHB,
I'd recommend a
Paris Visite 3 zone 3 day pass for use while you're in Paris, and to purchase single tickets for your trip from
Airport CDG to Paris and out to Versailles (8,40€ and 2,90€ respectively, one way, per person). The Paris Visite will allow you unlimited travel within central Paris and gives you discounts such as reduced fare on certain boat trips on the Seine. See the
Paris Visite page for more information on discounts.
Best regards,
Ben