Ideas still very welcome though
In Montmartre go get a Salade au Relais Gascon -they have two locations, the original Rue des Abbesses @ rue Hudon, the new one rue Lepic. BTW if it is a Friday or Saturday and it is about 9:00 PM, go to Cafe Hudon, they used to have live music, specially local jazz bands.
Near Opera Garnier or La Bourse, go get a Confit de Cannard (duck confit) au Lintillac (rue Saint Augustin, between AVE de L'Opera and Bourse)
If you are near Bastille, go to La Fee Verte (rue de la Roquette, 3 blocks from the Place de la Bastille) and say hello to Marieke, the owner. Lunch time is specially good, but just a coffee or a couple of beers is good also.
If you are still near Bastille, go to Boulevard Beaumarchais where you can find many camera stores (new and used)
From Bastille, walk to the Marais and stop at the Hotel de Sully, they have photo exhibits all the time (I saw Karsh and Eugene Smith there) Agathe Galliard's gallery used to be in that neighborhood also, but I think she had to move...
Last time I went to Paris (almost 2 years ago) i went for the first time to the Petit Palais and I was impressed and thankful (and wondering why I never went before...)
I can not tell about Calais (I went to London by Eurostar) or the UK in general because i only spent one weekend in London, about 8 years ago...
Enjoy your trip!