Not far from the Louvre on Saint-Honoré (about half way between the Louvre and Les Halles) is Boulangerie Julien. When we were there 4 years ago, they had an awesome lunch special - sandwich, drink and pastry for 10 euros. The line was out the door and down the street at noon, so plan accordingly.
Oops, I forgot my dos and don'ts.
Personally, I would skip Versailles unless you have a burning desire to see the fountains. It's a smaller, glittery version of the Louvre, overcrowded and will require planning to get there.
There's a ticket outlet in Les Halles (sorry, don't remember the name) where you can buy museums tickets a day or 2 in advance and skip the lineups. FYI: Museé D'Orsay is closed on Mondays and the Louvre is closed Tuesdays.
On Sunday, there's a street market behind Basilica Saint-Eustache. The fruit lady was memorable (in a good way) and we had figs that were the best I have ever tasted in my life.
And I agree with whoever said it earlier, do take the time to wander around some of the neighbourhoods and take a stroll down Champs-Élysées. Also, be super polite and try to learn a few opening phrases in French (Good day - Bon jour, etc.) to greet the shop attendant when you enter. Not offering a greeting is considered quite rude.