You might want to search/browse this forum first -- this topic is discussed a lot and you can probably find the answer. There also are several FAQs in the upper-right discussing transport from the airport.
If you still have question, you would need to provide more info. How many traveling, days of the week, where is your hotel/accommodation in Paris, etc. All of this information will affect what method you may be able to use.
thanks for your response..
Our hotel would be near the airport - Holiday inn express cdg airport.
We are 2 adults and 1 infant child (Age 20 months).Edited: 12 June 2018, 17:51
Hi, from your previous thread, it seems that you are staying at the airport due to a tour operator you booked with. As you are doing bus tours with them the rest of the time, a weekly pass like the Navigo is probably of no use. I recall this first day is your only independent travelling.
It appears your hotel is on the CDGVAL free shuttle train line to the airport easycdg.com/hotels-cdg-airport-de-gaulle/ which at least means easier access to the public transport.
RER train into Paris and back would be the cheapest option.
RER train from COG and then use the same ticket on the metro to transfer to the station nearest the site you want to visit.
€10.30 per single trip. 3 single trips & the Navigo Découverte will have paid for itself.
Google for Paris metro map and see for yourself.
On train strike days RER B will run less often and you would have to change trains (the same line though) at Gare du Nord.
As suggested earlier, the RER+metro is the cheaper option and yet it can be very expensive if you are not careful. Make sure to secure your money as we discussed on the other thread to avoid pickpocket on the train. Recently, there was a thread where the OP was pickpocket (cash, credit and debit card suddenly gone poof) on the RER from the CDG to central Paris while attending to his child.