We have 8 hours between leaving our apartment in the 3rd arr. at 9:30 and when our flight leaves from CIA at 6:00 p.m. Perhaps the apartment rental will let us leave a little later, but I don't think there is an option to leave our luggage there. Any suggestions about where you would go or do dragging 2 suitcases with you around Paris.?

Visit the Museum at centre Pompidou, it's near you and you can check the suitcases at the ticket desk (did that).

Luggage storage lockers at a train station? Or a ho/ho bus tour? They probably cost about the same.

I like the Pompidou idea. I could certainly incorporate that into just plain old walking around and eating somewhere. Thanks for both ideas.

yep, plenty of walking/eating around there, and you can live your stuff at the Pompidou desk while you're out.

I would pay for an extra day or negotiate to pay a half day if nobody else is coming in.