In context : From Rome to Babylone through Luxembourg
(From : Saint-Sulpice Church). If you want, have a drink on Saint-Sulpice square, then start walking across the XIIIth-century old 6th district : take Saint-Sulpice street, turn left into Mabillon street, along Saint-Germain market, up to du Four street ; then turn left again into Princesse street, then right into Guisarde street, then right again into des Canettes street ; finding back du Four street, turn left then left again into Bonaparte street. Almost back to Saint-Sulpice square, don't miss the tiny Pierre Hermé's pastry shop. Even if you don't buy anything, it will be a treat for the eye.
You'll probably have heard about French pastry chef Pierre Hermé's famous macarons, the most well-known being the Ispahan, made with rose, lychee, and raspberry. Hermé also makes chocolates.
Pierre Hermé's first shop was in Tokyo, and opened in 1998. The first Paris boutique opened 4 years later, here at 72, Bonaparte street. You should try it. Really.