While you might not want to stay on Rue Mouffetard, it is well worth a walk. Lots of food shops and restaurants and a lot of fun. In the area, I agree with the recommendation of the Musee National du Moyen age. You are also not too far from the Pantheon and Les Invalides (both Napoleon's tomb and a military museum). Really, central Paris is not all that big - you are not too far from most of the main tourist attractions and if you have never been there before, you really cannot come home without at least a quick trip to the Louvre, Notre Dame, Eiffel Tower, etc. I also recommend "Les Tours de Notre Dame", where you walk up to the top of Notre Dame and get quite a view. Depending on where your hotel is, you can walk to many sights if you are so inclined, as well as walking to the Ile St. Louis and Ile de la Cite (where Notre Dame is).

Have to agree with the suggestion to get a Museum Pass (they are available for either 2, 4, or 6 days) if you are going to go to more than about three museums (note that the Eiffel Tower is *not* on the pass). Another advantage of the pass is that you will not feel so guilty if you pop into a museum for a short while rather than feeling that you have to stay longer to justify the entry fee. You did not mention who you are going with or how old they are - under-18's get into most museums for free and do not need a pass.

Also highly recommend the Musee Carnivalet in the 3rd arr. (other side of the Seine from where you are staying), featuring the history of Paris and free to get into. But does close for lunch. There are a number of other worthwhile small museums in the area including Victor Hugo's house, the National Archives, and the Musee Grevin.

My favorite (so far) patisserie in Paris is right at the Cardinal Lemoine metro stop; do not remember the name but it has a orange/pink-ish facade (I remember pink, Google Streetview shows it at orange. Yummy nonetheless...) and one of the two entrances/exits from that stop is right in front of it (if you come up the other one, look across the street). The raspberry tarts are luscious!

If you like walking tours, look at Paris Walks and Classic Walks. Both offer good, inexpensive 2-ish hour walking tours on various themes.

For ice cream, everyone likes Berthillon, which is good, but my daughter and I are partial to Amorino, which is more of a gelatto place. For cupcakes, definitely Berko, up near the Pompiudou Center.

FYI, I have stayed in the 1st arr. near Les Halles, on the Ile St. Louis, and in the Marais which are all good places to say, too. Was thinking of trying the Latin Quarter or near the Eiffel Tower next time there, so your selection is a good one, too.

Have a great time! Unfortunately, I could not take advantage of this sale...