Definitely the Batobus.
We took our 7 year old last summer we only did two days, on route to Disney.
Day 1.
Start at Trocodero Metro and walk out from there for a wonderful view of the Eiffel Tower (prebook if you can, or just go up to the 2nd level, as the queue is shorter).
We then walked to the left of the tower towards the Seine and took the Batobus - the ticket allows you to hop on and off. We went down the river, getting off once at the Musee d'Orsay as she wanted to see some Van Gogh paintings!! We didn't stay long and had a free ticket too. We walked to a café and had a snack lunch.
Back on Batobus to Notre Dame and climbed up the top (the bells!!!), walked from here to the Pompidou Centre - very visual, interesting and street entertainers around. Had more food!
Walked (this was a mistake, it was long and hot), to the Louvre, but didn't go in).
Back on Batobus and got out a few stops along, walked up a shopping street (very posh shops, no idea what called) and then took the Metro to the Arc de Triomphe, it was now dusk and the lights were brilliant.
We then got back to our hotel, feeling we had seen loads.
The next Day, we did a different plan and went North to the Anvers Metro and got the funicular up to Montmatre, If you walk to the left and the back of here, you come to a wonderful area full of artists working. We had lunch in a pavement café, before wandering down to Pigalle and around the Mouins Rouges area.
In the evening we went to a local restaurant to our hotel and dd polished off a huge bowl of Moules et Frites.
We took it easier second day as we had an early train to Disney. That is sooooo easy to do too, the train takes you right into the parks and is worth a day. However, take sandwiches, as it is very expensive to eat here.
I wish we had more days, then we would have done the sights we did see and others in more detail.