Nothing too unsalubrious about the Line 11 extension or the Line 14 extension for that matter, but not really the best spots for a leisurely drink indeed. Les Lilas, where I live, its quite nice by itself, has bars and places to relax, but probably not the most enticing for a tourist.
Here's a possible itinerary:
- start at Porte Maillot and go west, to Nanterre-la Folie. Backtrack from there and go east to Rosny (30' travel time).
- There is a small railway museum at the Rosny-sous-Bois (not Rosny-bois-Perrier) station, run by an association:
http://rosny-rail.fr
It is only open to the general public on Saturday afternoons, though, so if your visit is on a weekday, you won't be able to visit. But their small collection of rolling stock is visible from the RER platforms
- reach Rosny-Bois-Perrier and change there to travel the Line 11 extension back to Châtelet