They are closing the roads alongside the river banks on Sundays, starting ??? until 5pm (don't ask me anything about Sunday mornings !), so it's a nice stroll with absolutely no traffic, few pedestrians and great views. Of course, there's the Sunday afternoon "randonn├â┬®e" starting at 2:30pm from the Bastille (lasts around 3h, moderate to slow pace, thousands of people).
I don't really see any other place within Paris where you can skate quietly off the road for a long while (I don't include here all the sidewalks, nor the slalom spots). There are a few bicycle lanes, but they tend to be on the road, and sometimes very narrow for a skater. One which is good is on the North-East, by the Grande Biblioth├â┬¿que, more or less following the railway tracks underneath (it's a newly built part of the city, quite nice and interesting).
Hope that helps !!