Don't forget gelato at Amorino - there are multiple locations.
L'as du falafelis great and reasonably priced - can eat in or take out.
We went for brunch at Angelina's - make a reservation as it's popular. There are several but I like the one on Rue de Rivoli near the Tuilleries.
There are grocery stores & boulangeries (bakeries) everywhere - Franprix, Carrefour, Monoprix and smaller ones. Fresh squeezed orange juice, cheese, baguette, wine/beer...yum. Grab a picnic and eat in one of the many parks, or down by the Seine. Carry a bottle opener, utensils, hand sanitizer. Keep in mind that while carrying a paring knife to cut the food on your picnics is a great idea, you have to check it before entering museums.
There are a ton of restaurants around Place Saint Michel - little ones that will make up your choice of crepe while you wait, take out falafel, or sit down ones. I like Rue Saint-Andre des Arts & Rue Saint Severin.
Walk the 2 islands in the Seine - Ile de la cite (visit the tip at Square du vert- galante and the bird market) and ile saint-louis.
There's some designer stuff at the Bon Marche and there's a separate building just for food/home stuff - also great for picnic supplies.
If you want cheap lotions/toiletries, Pharmacie de la Cite is near the Bon Marche. That whole area around Saint Germaine is fun to wander in, and the Jardin Luxembourg is also close by.
There's also Jardin des plantes where there's a good zoo and some museums, and that's close to Rue Mouffetard, good for a wander and some fairly reasonably priced restaurants with set menus (3 items ~20-30E) and touristy items like fondue and tarteflette.
Re: metro tickets, you might as well buy a book of 10 as that's 5 trips for the two of you and they're much cheaper than singles. Remember to keep your ticket as the guards check fairly frequently, and on the RER lines you need them to get out of the turnstiles. Apparently the tickets are supposed to allow multiple rides within a time frame, but I find once I leave the station I can never re-enter on the same one. The little offices in the metros have mini metro maps for free.
I haven't done a bus or boat tour so can't help there.
Have fun!!