Normandy at Easter

It is my understanding that Good Friday is just a regular workday in France; however, "Easter Monday" is usually considered part of the Easter holiday and apparently there are more things closed at that time.

Tara, is attire at Easter Sunday Mass in the U.S. usually "pretty casual" ?

Attire at mass varies, whether it is Easter, Christmas, or any day---from jeans (or even shorts) to what most would consider "Sunday best." I tend toward the latter. The church I attend now is on a university campus, so it may be more casual than most, but from my experience, I think people dress for mass more casually than how people dress for Protestant church services. Could just be where I live. Catholicism is more pervasive in Louisiana than other parts of the U.S., so that may be a factor.

Related to my question, "Sunday best" may not make it into my suitcase, so I was trying to find out what would be acceptable middle ground.

Well, since you will be in northern France and the weather will probably be cool and maybe wet, you can probably wear black (or other colour) pants, a sweater or top of some kind, and a nice big shawl worn with panache. No-fail outfit.