The Batobus is a bus-boat in Paris. It's possible to hop on and hop off at any of 8 stops along the Seine.
You purchase a day pass at any stop which allows you to get off and visit say, the Eiffel Tower, and then hop back on and ride down the Seine until another sight strikes your fancy. Or, you can just sit back and enjoy the ride like this passenger:
The dog did manage to see a few sights along the way: