Paris is a place that really is dependent on your tastes. I went on this particular trip with a large group of people--about half of them leaving the city telling everyone it was the best place they'd ever been, and the other half saying they hated it and wished they could have spent more of that time in another city.
But whichever side of that argument they fell on, pretty much all of them agreed on one thing: the boat ride down the Seine was one of the best, if not the best, things they did while in Europe. I think that Paris is a city like Barcelona, where you really need your own time and pace to appreciate the city as you see fit. If you get rushed through to all the museums and the sights, you really do miss something, and as it's a busy and crowded city anyway, it begins to become a hassle rather than an adventure. But whatever kind of trip you do find yourself on when you reach Paris, this one bit is something you can enjoy at your leisure.
There are lots of boats and tours to choose from; I would recommend checking the Paris Tourist website for prices and times. The one we took left just after sunset across from the Eiffel Tower. I rushed onto the boat after taking a frantic roll of film at the Eiffel Tower, sat down with a sigh, and realized I'd not sat down in almost 20 hours. And then I looked up as the twinkling lights of the Eiffel Tower began to light up and the lights began to glow on the water up and down the river, with soft French music playing softly while people loaded onto the boat, and then I did something I almost never do: I put the lens cap back on my camera, put it in my bag, and leaned my head on my hands to enjoy the ride. Some of my friends came back from that trip with stunning photos from that boat ride, but I don't regret my stopping to enjoy it for a moment.
It's a quiet 1-hour ride where you get to set yourself apart from the bustle of the city for a moment and watch it in luxuriant passing. The buildings you pass along the way are stunning, with what could only be French architecture; the music plays softly; and you get to see the locals walking arm-in-arm along the water's edge without having to edge past them. It was stunning, and something that a simple 500-word review could never do justice. If you're in Paris, don't miss this bit. It's something you won't ever forget.