CDG is a bit more simple since it is located to the northeast of the city.

For CDG:
Sit on the right side of the plane when landing. If you are coming in from the west, you might get a tremendous view of Paris especially if you land on the southern set of runways. Coming in from the east, you aren't likely to see anything when flying in from LHR so it doesn't matter.

Sit on the left side when taking off. Again, if you are taking off to the west, you will get a good view of Paris but you probably won't see a whole lot as you will soon make a right turn heading north.

Orly is more complicated since longitude-wise it is pretty much the same as Paris. Since aircraft normally land from the east, I would recommend sitting on the right-hand side although the views aren't super great. Same for take-off.

If the winds are the other way, you would want to sit on the left side when landing (you'll easily see the Eiffel Tower even on the ground at Orly) and right side for take-off since the plane tends to make a 270 degree turn and then flies directly over Paris and the views are absolutely tremendous.

Sorry for a super long answer, but hopefully you got something out of it. Of course, this is assuming it isn't cloudy in which case you aren't likely to see anything.