I went to St. Denis on a Friday and there was a huge outdoor market in the square between the Metro station and the Bascilica. I don't know what other days the market may be there, but you can try Googling "St. Denis market" and see what else comes up.
In addition to St. Denis that day, I went to the Musee Nissim de Camondo. I transferred from line 13 to line 2 at Place de Clichy, direction Porte Dauphine, and got off at Monceau, which is right outside Parc Monceau, which is one of the most beautiful in Paris. Another possible place to go from St. Denis is Montmartre. Transferring to line 2, direction Nation, you could get off at Pigalle. Using the Metro Map opens up lots of possibilities for planning your sightseeing day. Get a Plan Deux map at a Metro info booth. It has the Metro on one side and the buses on the other, each big enough to read. It's great for planning. Have fun!