I love Paris. I spent a year there as a student, but was far too poor to make the most of it. Have been back a few times since, either for work or on personal visits, and each time I expect the novelty to have worn off, but it doesn't.
You don't have a lot of time. I'd suggest getting yourself a good city guidebook and have a read up of things that interest you. Definitely research the Louvre. It's enormous and it will really help and save you time if you have an idea beforehand of which items/galleries you want to see. You'll probably have to queue to get into the Mona Lisa gallery (assuming you want to see it) so allow some time for that.
The Eiffel Tower - I guess it's a must-do on most first-time visitors' lists. I think it's quite an ugly structure personally but the views are good on a decent day (you can also get good views from the Arc de Triomphe and Tour Montparnasse, with fewer people. The latter is a bit out of the centre so probably not best use of the time you have). Buy your ticket in advance as the queues can be insane (although you probably still have to queue even with an advance ticket, so get there early). I think there are a couple of restaurants in the tower, which if you are booked in (e.g. for lunch) mean you can skip the queues I think. Haven't done it myself so don't rely on that without checking.
The approach through the gardens and fountains in front of the tower (Champs de Mars) is pretty.
Given that you only have effectively little more than a day, from the time you are checked in to your hotel to the time you have to pickup your bags and head back to the Eurostar, I'd spend most the time (aside from what you already have planned) just walking around the city. It's actually pretty compact and you can cover a lot of ground in a day. Just make sure you both have comfortable shoes! Like London it's a city worth seeing from above ground, not on the metro. Go on to Amazon and get yourself a scale city map - don't rely on getting anything decent from your hotel. Lovely buildings, architecture, and side streets made for wandering. Stop for a coffee here and there and watch the world go by. Beer is expensive mind.
@SpurMeUp has made some good suggestions for wandering around the centre (Rodin Museum is indeed lovely if you like sculpture and the nearby area around Les Invalides is a nice walk up to the river from where you are staying), as has
@braineclipse with Montmartre and the area around the Sacre Coeur, if you can squeeze them in, or if not, save them for your next visit (Montmartre is a good place to stay and has some nice restaurants - just don't wander too far into Pigalle!).
Do either of you speak any French? If not, download a translation app on to your phones. Unless anything has changed, there is some truth in the reputation of Parisiens not being particularly helpful to those not being able to attempt to speak French (not in main tourist places obviously).
I am sure you will have a wonderful time and will want to go back again.