I have done London - 1 night and, another time, Paris - 3 nights. These posts give good ideas. The bottom line is to do what you like best. Like shopping, then do it without guilt. Like particular kinds of art, then do it. Like architecture, then look for your favorite buildings or fixtures. I like commercial aviation so I went to see the Musee de l'Air et L'espace (Air & Space Museum) at LeBourget Airport. Accessible by bus from Gare du Nord and CDG (171 bus??), it has plenty of military planes also. The museum is located in part of the old terminal and several hangers.