What time do you arrive? You can at least drop your bags off at your hotel as soon as you get in, even if your room isn't ready.
Starting from your hotel, I would walk down to the Seine, and head for Notre Dame. Depending on your energy level, it's well worth the walk along the Seine (maybe have lunch around here), past the Louvre (even if you don't want to go in), through the Tuileries, Place de la Concorde, then up the Champs Elysees. At any point, if you feel so inclined, you can cross the Seine and head over to the Eiffel Tower (but you'll be able to see this along your walk). You can even head as far up as the Arc de Triomphe. This total walk is probably 1.5 hours of actual walking, but you can (and should) obviously take stops to see and do stuff along the way.
When it gets closer to dinner time, I would take the metro to Montmartre, see Sacre Coeur, and find somewhere to eat in that area (lots of good places just west of Sacre Coeur). Walk around Place du Tertre. See the sunset from up here Montmartre is on a hill). Take the metro back to your hotel and crash.
Again, this depends a lot on when you arrive and your energy level, but I am usually raring to go when I land. Have a great time!