I don't want to put a downer on the bus suggestion because it looks very cool but I've just spent 6 months living and working in the areas that bus runs through and I never saw one in that style. They may have finished the service. Definitely worth checking before planning.
On a more positive note, go to Harrys Bar on Rue D'Anou just down from Opera Garnier for a drink, amazing interior, been there for ever. Hemingway drank there.
I had planned to go on the guided tour of the Opera Garnier and when i saw the scale model in the Musee D'Orsay I realised the mistake I'd made, it looks well worth the visit.
There is a tiny museum opposite the Musee D'Orsay all about the Legion D'Honneur, which is presented to people from all walks of life, not just military. Free to get in and ignored by the vast majority.
If you are staying near the Gare du Nord then there is a covered market near Gare de l'Est Marché couvert Saint-Quentin that has some lovely food stalls.
Whilst on the subject of food there is a cluster of shops at the bottom of Rue des Martyrs at the junction of Rue Lamartine. Cheese, fruit and veg, rotisserie, more cheese and many other things. Depending on where you are in relation to the Gare du Nord then it may be within striking difference. If you could say I may have some more specific suggestions.
Also on Rue Lamartine at No. 7 is an amazing Turkish/Greek/Persian/Moroccan style deli/market thingy. You won't get dinner there but you will pick up some nice pastries, sweet and savoury. Baskets full of nuts and spices and all sorts.
Finally at the top of the Musee De L'Armee in the attic are a load of huge relief maps that Napoleon had made for his various campaigns, really detailed, very large. Some of them are bigger than flats I've lived in.
These are my recommendations, there may be better but this is from my personal experience and I like them.
Finally, the best bread I have ever eaten is from a bakery at the top of Rue d'Athenes near the junction of Rue d'Amsterdam. they sell a thing called "Crockys" and if you like your bread crusty, light and just on the right side of burnt you will die happy.
Extra finally, don't go to the aquarium, it's shit.