We arrive in Paris on Fri., April 15, around 10a.m. from the US, if the gods are smiling. What's our best way to get to the upper 15th without a gazillion stairs? Closest Metro station is La Motte-Picquet Grenelle. Open to train/bus/Metro/combo; quick travel time isn't primary (though nice, natch), but ease with baggage is. Cost's importance is somewhere in between - we typically pinch pennies, but with bags & tiredness, not so much. Thanks!
I suggest a taxi for this trip. It is a fixed price of 58€. Go to the taxi stand. Do not accept offers for rides from anyone who approaches you. Write the exact address including the postal code on a piece of paper along with the price and hand it to the driver after saying bonjour. By mass transit, take the RER B from CDG to Denfert-Rochereau and change there for Metro Line 6 (direction Etoile) and exit at La-Motte-Piquet-Grenelle. There are other routes. This one and all the others that I know of will require you to manage stairs. Not a gazillion mind you but at least a flight or two. At Denfert-Rochereau, for example, the RER is below ground and Line 6 is above ground. I think mass transit or a taxi will take roughly an hour.
I agree with JHK. This is one of those times that a taxi is called for. If you want to avoid stairs (understandable with baggage and fatigue), that change at La Motte Picquet is not one you want to do.