It would be a waste to walk by the Opera at night, even though it is beautiful, the area is dead at night, unless you go by Les Grands Blvd, but why waste your time. The 42 bus would pass by there, just look at your left and you will see it.
Bakeries close at 7ish. So your best bet is the am, and each area/person has their favorite bakery. It depends on where you will be staying.
You can see Notre Dame, but really walking the city after your dinner is the best thing. Just walk along the river and you will have the best views ever.
As for storage places, I really can't help you there, since I don't need to store.
But since you will be (am assuming) staying at a hotel, how about if you leave your backpack there?