There is a Renault Museum on the Champs-Ellysses that is interesting, there are a number of older race cars. I thought it was cool and I am not a car person. There is a huge Adidas store near there too.
The view from the top of the Arc du Triomphe is really marvellous. I must admit that I did not walk up the stairs; I learned the words for I hurt my knee and used them at the elevator. Just to see the crazy traffic is something. Rick's book has directions to get to the entranceway which you must access underground.
Watching the sunset over the Eiffel Tower from the Trocadero (other side of the Seine) and seeing the first Twinkling lights of the evening of the Tower on the hour to 5 past the hour. Lots of people hang around waiting for this. One of my favourites.
A boat tour of the Seine at night. And maybe during the day too.
Have a picnic in a park or along the Seine, outdoor markets have premade food as do many shops. Grab a baguette with ham and cheese and find a bench along the Seine. Heaven.
Not sure about a 17 year old boy liking open air food markets but they are great. Chickens with heads and feet on, maybe a whole pig, octopus, whole fish, I love it. You can buy your cheese and bread there for your picnic.
The huge clocks in the Musee d'Orsay are fabulous. My favourite museum. Not to big, cannot really get lost there.
The Pantheon has Foucault's pendulum and tombs underground. I have only visited once and was quite taken by the place. A beautiful building.
Eat dessert all the time, you will walk it off. Have flaming crepes Suzette, ice cream ( my fave is Grom) & Amorino. At Cafe Constant have a dessert of cream puffs where they pour chocolate sauce over them! It is near the Eiffel Tower and in Rick's book; I love that place.
One of my favourite market streets is rue Mouffetard; many bakeries and food shops. Near there is the Jardin Des Plantes which is lovely.
Notre Dame Cathedral, the park at the back is very pretty. Look at the gargoyles, even from the ground they are great. Viewing the cathedral from the back from the opposite side of the river is wonderful; I cannot remember the name of the bridge to view from. On weekends there might be musicians around. Be careful if pick pockets there. Both the islands are lovely to walk through. Bertillon ice cream shop is on one of the islands.
Watch the first Bourne movie before departing then visit the Pont Neuf or view it from either side of the Seine.
Walk right along the river to look in the houseboats.
Is that 10 things?!? Paris is my favourite place, I will return this fall. Have a great trip.