The best way to get to Ris-Orangis from Paris, is via Taxi I was told . Not so true. Firstly, it is costly, especially, if you have to do it every day, back and forth for four days. Secondly, it is boring. There is lots to see on and outside of the public transit --and if there is not, one can sleep, read or listen to music.
A 5-days pass for zones 1 - 5 costs 67,40 € (2015, October). And from anywhere in Paris you can get to Gare Du Nord or Les Halles Stations where you can catch RER D train operated by ROVO South (Sud) to CORBEIL ESSONNES. I caught this at 7:42 AM and was in Ris - Orangis at 8:20 AM.
There was no cab at the train station, to my surprise. So, I had to try my French in getting the point across to the attendant at the train ticket station, who kindly made a call on my behalf. The taxi however arrived in sooner than the expected 15 minutes. I tried my French again on reserving a taxi for tomorrow but I gathered from what the driver said, I have to call again tomorrow. By end of the week the attendant at the train ticket station will either avoid me or we become good friends. I certainly hope for the latter to happen!
Going back to Paris, I took RER D train North (Nord), at 5:31 towards VILLIERS LE BEL GONESSE ARNOUVILLE, operated by VUPE, from ORANGIS BOIS DE L EPINE, a different station that the one I arrived in the morning, and got to Paris around 6:10 PM.
There are many trains going through the same station, but they go one at a time :), so for the information about the next train approaching are on the monitors, this includes time of arrival, and all the stations the train stops. This is truly a source of reassurance. However, if the system is down, monitors showing a blue screen, it's best to know the train information, timetable, and operator from a previous time, basically get on the train that arrives at that time, and listen to stop announcements, those French statements starting with: Prochain arret ....., to ensure you are on the right one.
The following two websites are source of reference for planning to visit Paris and/or Paris and the vicinity.
Paris and suburbs: http://www.ratp.fr/