I also vote for a bus tour but NOT the hop on hop off bus. We found it very frustrating wasting so much time at each stop. We also love the boat tour especially at night.

I second "Valtor's" recommendations for the day; we have been to Paris 5 times, each about 4-5 days, but the biggies are all covered in the above list. Good luck! It is a very walkable city and I have never ridden the buses.

Sweetness, please come back to this thread and tell us how you spent your day.

Bon voyage!

In my opinion Valtor's itinerary on the day you get off a plane is impossible. It's enough for two days. Just MO.
Perhaps the idea of staying at the aiport is good. THEN I would take a taxi into the city and go to the Eiffel Tower. You can buy a HOHOff bus ticket there. Just do the main tour, not the add ons, unless you want to do more. Get off at Notre Dame and walk around that area.
Plan the Seine River cruise from the Pont Neuf. Take a bottle of wine and glasses on with you--buy it at a shop, not at the boat bar. The cruise is an hour, and if you do it just at dusk you can see the lights come on and then by the time you get to the Eiffel Tower, the lights will be twinkling on it.
The DK Eyewitness Guide to Paris will give you good ideas of things within areas of Paris, and where you might like to walk and enjoy.
If you go to the Arc de Triomphe, you can get on a bus that will go all the way down the Champs for a view of the avenue. It turns across the River at Pl. Concorde, so you can either get off or continue over.