Hi Vicki,
The most cost effective ticket solution depends on your style of travel/tourism and if you'll be visiting sights outside of central Paris (such as Versailles).
If you like to walk a lot & staying within central ParisAs you're arriving on a Thursday, I would suggest buying return (aller-retour) [urlhttp://parisbytrain.com/billet-ile-de-france-train-ticket/]Billet Ile-de-France tickets[/url] for the
RER B to get in and out of Paris (4 tickets x 8,40€ = 33.60€, I'm assuming you're two people). Keep in mind that these tickets will work for connections on the Metro as long as your trip is uninterrupted (thus you can get right to/from your hotel using a single ticket).
Two carnets of Metro tickets (20 tickets for 22,80€) shared between you could cover your travels within Paris. That's basically 5 return trips from your hotel to somewhere in Paris, for each of you.
Total cost: 56,40€
Option 2: Prefer taking transit a lot & are staying within Paris Again, buy billets Ile-de-France for the RER B from/to the RER B as before (33.60€), but also buy a couple 5 day 3 zone Paris Visite cards (2 x 28,30€) which will cover all of your travels within central Paris on all types of transport, cheaper than you could do so with 5 Ticket Mobilis day tickets.
For the last day before returning to the Airport you could either buy a couple 2 zone Ticket Mobilis (2 x 5,80€) for unlimited travel on Dec. 10.
Total cost: 33,60 + 28,30 + 11,60 = 73,50€
If you're planning visits outside of Paris, you'll need to weigh whether it's worth it to buy a 6 zone Paris Visite (48,80€ each) to cover that, or whether it's more cost effective to buy single billet Ile-de-France tickets for travel outside. The Paris Visite is obviously less hassle, but a waste for making a single trip outside of zone 3.
In the above I'm considering that the first and last days are mostly spent on recuperating and getting to/from the airport which limits tourism possibilites, so you may need to adjust my suggestion...
Best regards,
Ben