I did a lot of research about this to make sure it was a good day trip - it was a little further than I like to go. I usually will go up to one hour each way for a day trip. Nancy is 90 minutes each way but I am happy to report it worked out.
We took the 9:15 from Strasbourg and got to Nancy at 10:45. We then took the 3:15 train back to Strasbourg, arriving back at 4:45.
Between 10:45 and 3:15 we walked about 8 miles and covered this:
Walk from train station to Nancy cathedral for a brief walk through
Walk to Stanislas Square to wander around - did not go in any museums
Walk over to the Basilica of Epvre, sneak into the very small entryway to see the GORGEOUS stained glass
Walk over to the Ecole de Nancy to see the museum, with its great furniture and fixtures
Walk over to Villa Majorelle with pre-bought 2pm entry tickets to tour the house (very limited areas to see)
Walk back over to the train station - arriving at 2:30.
So can it be done in a day? Absolutely! We had a great time. Nancy is fantastic!!!! I think that if you took the 5:15 train back you could also sneak into the Beaux Arts museum for a bit.