Lots if great reviews for Paris Perfect. We used vacationinparis.com, based in the US. One great advantage to an apt is having a kitchen. Plus living among locals. And not needing to leave the room for maid service.
Also look at the free guided walking tours led by volunteers. We did a night tour of Ile de la Cité. Lots of choices and look at TripAdvisor for reviews.
After all the usual highlights, this was one of our best experiences.
Yes, check the weather. Off season is great.