Best way to travel to Versailles & Disneyland Paris

How long are you in Paris, and which days of the week?

A Pass Navigo is a Monday through Sunday transit pass that is good for the whole Paris region, including the airports, Paris Disneyland, and Versailles. It requires a photo, and you each require your own Pass Navigo. If your travel is over these dates, it would be a great deal for you. Again, it's Monday through Sunday, not any seven days.

If the Pass Navigo won't work for you, then you can buy regular tickets. Beware that a carnet is just 10 tickets bought together at a discount; any kind of ticket can be bought as a carnet. When people speak of carnets, they're usually thinking of T-1 tickets, which are only good for the metro within (and sometimes a bit outside) Paris itself. A T-1 ticket is not valid to Versailles, Paris Disneyland, or the airports. You will be able to get into the system with a T-1 ticket, but you won't be able to get out at your destination, and face a fine for traveling without a proper ticket. If you are using T-1 tickets to get around Paris itself, you should definitely buy a carnet of them, since it's cheaper than single tickets. For Disney, Versailles, or the airports, you will then need to buy a separate ticket each time, to that specific place. Since these tickets will look similar to T-1 tickets, be sure not to mix them up.

For all trips, hold on to your tickets until you're outside the system. Not only are there ticket checks (again, with fines for those without valid tickets), but you will need your ticket to exit or transfer if you're using the RER (the trains to Disney, Versailles, and the airports; they've been officially renamed, but at least in May 2018 when I was there, the RER signs were still there).

A carnet is a discounted pack of 10 tickets of the same type. You need to purchase special point-to-point tickets to go to Versailles and Disneyland, since they are outside of Paris proper.

Speaking of Disneyland next month, the RER A, which would normally take you directly there, is shut down in central Paris from July 28 to August 26. Track construction. So you will need to research other means. Use the RATP planner.
https://www.ratp.fr/en

If you are going to be there for a calendar week (M-Su), a Navigo Decouverte one week pass is a good deal. Costs about 23 EUR plus a 5 EUR set-up fee. Bring 1" by 1 1/4" passport type photos. That will cover just about all transport, to/from the airport, Versailles, Disneyland and within Paris. It always expires midnight Sunday, so it works best if you are arriving early in the week and leaving on a Sunday.
https://www.ratp.fr/en/titres-et-tarifs/navigo-monthly-and-weekly-travel-passes

Wow this is great info, thanks! I had no idea about the RER A being closed! Versailles travel is on a Friday & Disneyland on a Sat, we are only in Paris a few days. We arrive on Thursday morning. Sunday morning we leave for Venice. I believe that the Navigo week pass is best for our needs. Is that something I should order online ahead of traveling? Or buy it once we are there?

Buy once you get to Paris, since they have to attach your photos to the pass.
Looks like you can pick up the RER A at Nation. RATP suggests walking to the Metro No 6 line at one of the stations in the 6th and taking that to Nation.

Get the ND on arrival at the airport RER office or train station if you arrive there. Navigo Decouvertes can be loaded with a pass for the current week through Thursday. If loaded Friday or later, it will be for the next week. You can use the ND to get from CDG to your hotel in the 6th, and also to Disney and to Versailles. Be sure you have a timed ticket for Versailles as the security lines without that are a nightmare. This will be far the most economical transport option even if you decide to come in from the airport in a cab, as long as you buy the ND on Thursday you will be able to use it the rest of the week.

If you arrive on Thursday and spend Friday at Disney and Sat at Versailles and leave Sunday then you have no time for Paris. If you have not seen Paris before then this seems a shame.

For Disney, you need to get to Nation metro/RER stop to pick up the RER A to Disney if you are traveling during the time the RER is down in Paris. The metro lines 1 and 9 go through Nation and the 2 and 6 terminate there so any of those metros will get you there. There are undoubtedly bus routes as well that would get you there.

Not going to suggest dropping Disneyland Paris since many suggested it to you in February. If you buy the Navigo at the airport on Thursday morning and use RER to get into Paris and back out to CDG on Sunday, you will save money even if you only take ONE trip outside Paris (Versailles or Disneyland.) If you have the Navigos you will NOT need a carnet to go around central Paris -- and you can save the Navigos and recharge them for another trip to Paris.