I'm staying at the Hotel Auteuil Tour Eiffel for a few nights in July. It is the the 16th arrondissement. I want to take a trip to eurodisney on one of the days. I assume the best way is the metro RER from doing some research but I'm not sure of how to actually buy the right ticket for return journey. Each website I check gives different prices and suggests coins to buy? Please help as I am a single adult travelling with two children
I'll provide my recommended route first, then tell you about the ticket.
Walk from Hotel Auteuil to Passy station then take Metro line 6 (direction Charles de Gaulle Etoile, which I'll call "Etoile" from now on), until the end-of-the-line/terminus station, then transfer to RER A (direction Marne la Vallée Chessy) & ride until to the end of the line, which ends at Disneyland Paris. (Google maps route for Passy to Disneyland.)
On the RER A train platform at Etoile, only board a train that is bound for Marne la Vallée Chessy, and not Boissy Saint-Léger. There will be overhead signs on the platform showing which stations are served and the terminus, which you'll want to see "Marne la Vallée Chessy" (or abbreviation thereof) as the terminus.
I'd recommend buying basic paper RER tickets from the personnel staffed ticket window (not a ticket machine) at Passy Metro station and let the ticket agent create you the proper tickets for "Disneyland" for you. It'll be a single ticket for each of you & will cost 5€ each person, each way. The agent will likely ask "Aller - Retour? / Return?" which if you respond yes, she'll just print two tickets for each of you (so 30€ altogether, return. Cash, coins, credit card should all be OK). These tickets never expire so you simply use the 2nd ticket whenever you want to leave Disneyland.
These tickets will be good for both the Metro 6 from Passy to Etoile and the RER A from Etoile to Marne la Vallée Chessy (i.e. Disneyland's train station). So when entering the Passy Metro station, use these tickets in the fare gates/turnstiles and collect it to use for the remaining journey. Just keep the tickets with you until you're out past the fare gates at Marne la Vallée Chessy (you'll need the tickets to open the fare gates at Marne la Vallée Chessy). Once you're clearly out of the train station and entering Disneyland, trash the used tickets so you don't get mixed up between the tickets you've used and the ones for your return journey.
For the return journey you'll run this route in reverse. At Disneyland, the RER A trains will all say "Paris" as this is the terminus/start of the line. You can't take the wrong direction. When the RER A gets to Arc de Triomphe / Charles de Gaulle Etoile station, get off the RER A and transfer back to Metro line 6 and board any train that is waiting there at the station. Etoile station is the terminus of Metro 6 so you again, you can't take this line in the wrong direction from Etoile. It'll be going in direction "Nation"
On top of it being impossible to miss your transfer at Etoile, Etoile station is very well marked for the RER A train to Disneyland. When you're transferring from Metro 6 to RER A at Etoile, you'll see lots of signs with a Mickey Mouse silhouette on it which makes it easy to follow and to know you're on the correct RER A platform. Also, bus transfers are not included on RER tickets, hence my suggestion to walk to Passy. And finally, the other routes using different RER & Metro lines have trickier/more complex transfers. This Passy/Etoile/Marne la Vallée route is simpler/easier.
Have a great visit.