Day trip to Paris in late September

Hi there!

I have just booked tickets on the eurostar for my partner and I to go to Paris for the day - only time we could fit it in unfortunately. So I was wondering if I could get some inside knowledge on what are the best things to do on a day trip - get into the platform at 9am, leaving 7pm. I was thinking a bus tour might be the best thing to do initially but am confused as to which one is the better to do. Any ideas or suggested day plans would be great! We both love sweets/chocolate so any thing that involves that would be great too!!

Thanks!