RER B from CDG into town is easy. You can get the 24-hour pass for zones 1-5 that will cover your transfer to ORY that night as well.
Left-luggage services are available for the public in Paris railway stations : Gare du Nord, Gare de l’Est, Gare de Lyon, Gare d’Austerlitz and Gare Montparnasse. It'll cost you a few euro to secure the stuff in lockers but it works just fine.
https://en.parisinfo.com/practical-p...ilway-stations
Gare do Nord is on the RER B line that you'll take between the airports. Maybe drop your bags there then head a little further "in" to town for the afternoon. Return in the evening to wrap your day from Sacre Coeur (10-15 minute walk from the station) before collecting the bags and heading to ORY.