Galerie Documents, vintage poster shop, 53, rue de Seine. I never go to Paris without stopping in. Mireille (owner) and Dominique have a story to go with every poster. You can trust that everything is an original.
Cafe Charlot, 38, rue de Bretagne. Very good inexpensive food, great people watching, excellent creme brulee
Blé Sucré, 7, rue Antoine Vollon. Best Kougn Amman on earth (if you're not familiar, think carmelized butter and sugar between layers of puff pastry). Go early. They run out. If they do, you'll just have to settle for a croissant (also the best I've ever had, especially when they're still slightly warm).
Grand Gallery of Evolution at the Natural History Museum. Not really off the beaten path, but no one ever talks about it and it's magnificent. It's what Noah's ark must have looked like.