Gosh......romance can be the most simple things in life........small little things that happen with great sincerity that demonstrate your love. It really is the LITTLE considerate things.
You have some GREAT ideas already......walking along the Seine, or just over one of the lovely bridges. Evening cruise (open air seats) with the lights on the Eiffel twinkling is good, assuming a lovely evening weather-wise. Chilly?, take off your jacket and warm her...and/or stop and buy her a fabulous Parisian shawl.
Surprise her some morning (or every morning) by bringing her orange juice and a chocolate croissant (assuming she likes those...or a muffin and tea...whatever she likes....it can be from the hotel's complimentary spread...it does not matter) while she gets ready for the day.
In a quiet moment, tell her how wonderful your life is because you are sharing it with her.......ditto for how this trip is your favorite trip to Paris, for that same reason.....then take that selfie of the two of you at that moment (or get the waiter or another traveler to do it, if you cringe at the thought of selfies...LOL). Take the initiative yourself to have the photo printed and framed when you get home.
Put a specially selected card on her pillow in the evening with a handwritten, from-the-heart little note.
Agree with other posters, it depends on what SHE likes and wants to do...but if she does not offer up ideas, take the lead a bit. If you are going somewhere you have been, but she has not, introduce it that you want to show the most special person in your life what you think is the most special place in Paris....if you can pull that off without sounding scripted or cheesy.
A toast to each other over dessert at a small outdoor cafe. Or, when you are walking along an outdoor market street, stop and buy her a few flowers as a surprise on the spot. If you take home a bottle of French wine, plan to open it exactly a year later or on your next anniversary (then make a note to do just that).
Select a special Christmas tree ornament together (that you buy in Paris), and then each year when it is placed on your tree, you can remember your special trip together.
When those "cranky moments" of overseas travel happen (and they do when one travels with someone else....travel often brings out the worst in us with the long flights, lack of sleep, etc.), look at her and tell her she is beautiful and that you love her.
Even though you will travel carry-on, carry her bag for her and ditto for in/out of the overhead.
The big, planned romantic things are nice, but the little, sincere actions are what will make the trip :)
Note: advice from someone who recently celebrated 46 years of marriage..