Lucky you Breton!
I've spent more time in other parts of France, more in the countryside, Normandy, the South, Paris was always rushed, with friends taking me here and there, and me not recalling the names of places I had that great cassoulet, etc...
But I was thinking an affordable thing to do is hit some nice bakeries, sample some of the more renowned breads.
You could pick up a Pain Poilane (miche)
Bienvenue chez Poilâne in your first couple days, they're big and keep long, and you'll have something around. I love grocery shopping in France, simple things there like the array of olives at the olive counter, or being able to get a good duck breast or rabbit tenderloin or such, just in a basic supermarket. Great wines for much more affordable prices than here.
Eric Kayser's bakery is a must also (rue Monge), for some levain croissants, buckwheat batard, whatnot. He and Alain Ducasse co-own a "deli" BoulangEpicier
Boulange épicier in the 8th. I've never been there but sounds like a must go to place, and you can pick up some nice groceries too.
I also like Bernard Ganachaud, he's MOF (meilleur ouvrier de France)
You could hit a choclatier or two also. I always love finding unique cheeses when I travel also.