We're arriving at CDG at 12:55 on Friday, August 16 and leaving the following Friday, the 23rd. What would be the best combination of single tickets and passes to buy, considering we'll make 2 trips on the RER B train plus the subway mainly in zones 1 - 3?
Get a Navigo Jour day pass for use on Friday arrival, 1-5 zone (gets you from CDG to city + anything else that day), Navigo Jour day pass 1-2 zone active for Saturday for use in Central Paris. Buy the Navigo plastic card itself plus the two day passes, plus a Navigo week pass (1-5 zone) all at the same time upon arrival at CDG in either T1 / T2 train station ticket office.
This still leaves Sunday uncovered. On Saturday you can return to any Metro/RER/Transilien ticket office in Paris and buy another day pass for use on Sunday. You can't buy 3 day passes at the same time since a Navigo card can only hold 2 day passes on it at any one time.
The Navigo Week pass you'll use Monday-Friday and it will be good for you to return to CDG on Friday the 23rd.