I spent a quick 8 hours there back in 2012 and did the following -
Hop On Hop Off Bus tour - Fantastic idea, takes you to all the tourist parts and loads of buses so no waiting around.
Louvre - spent about 2-3hrs there, just enough to see the main attractions but all 1 billion Chinese tourists were there so I left ASAP, wish i had more time/less people there, a big highlight though.
Trocadero - great views of Effiel Tower without the huge queues to go up it. Plus views leading away to the Champ de Mars where everyone has lunch.
Notre Dame - great views inside, probably not so much now.
Seine tour - Just a quick 1hr blast up/down the seine. Great views but a bit smelly strangely.
Arc de Triumph - Quite small, on a very busy road/roundabout so access was tricky but great history on the walls.
Overall the bus tour was absolutely fantastic as a quick way to see everything , you got to see so much but the lack of time meant everything was a "quick look" rather than a slower "take it all in" which, TBH, is what it really needs.