Do I need a transit visa to leave the airport and spend the night in Paris?
No, you don't need a special visa for France. Both France and Greece are in the Schengen area.
Can I check a bag through or must I retrieve my luggage and go through immigration and customs?
That's kind of up to the airline/airport. Especially for long, overnight layover you'll often have to claim your luggage. They will of course tell you when you'll drop off your baggage in Athens. But in my experience, there doesn't seem to be a fixed rule on how this is handled. But for a 20h, overnight layover, I would say it's more likely than not that you will have to retrieve it at CDG. However, CDG has storage for luggage (as everything at the airport: it's a bit pricey).