If you want to be treated well in Paris, wear jeans and carry a backpack. The rational frenchman is supposed to be a little carzy when young, to get it out of his system and, consequently, the French are quite indulgent toward their youth. Correctly camouflaged, you can watch your hosts practice their favorite contact sport, being rude to affluent looking Americans. There used to be a good Vietnamese restaurant on the square directly across from the main gate of the Sorbonne; let us know if they still brew the tea with hashish.