Yes, unless you are travelling on a passport that requires a visa for entry to Cda, you can leave the airport.

The only tricky part will be ensuring that your bags are through-checked to Paris. This is certainly the norm when departing from other US cities, so I don't see why it wouldn't also be true of Charlotte.

You will proceed through the intl arrivals area, ignore the sign for intl transfers (customs B) and clear regular CDN customs and immigration (customs A). Enjoy your lunch in Toronto. Then return YYZ and go through the intl departures security.