Go to Promenade Plantee
This is one of the jewels of Paris. What was a liability has been transformed into an incredible asset. As one walks along the elevated and landscaped portion a completely different view of Paris is seen one story above street level. Architecural details are much more apparent and as one goes from its origins near Bastille, and goes to the outer edges of Paris the architecture changes from Hausmann to starkly modern. It eventually goes back to ground level and comes out in a neighborhood near the Peripherique, which, should you want to go over to the Bois de Vincennes, you must cross. This is the complicated part of the journey, and it might be just as well to visit the Bois separately.
Several years ago a friend and I traversed the entire length of the Promenade, enjoying the way the garden evolved as we strolled along. It is one of my favorite memories of that trip to Paris. (Note: it doesn't go directly to the Bois de Vincennes.)