We will be arriving in Gare de Lyon from Geneva around noon on July 13, 2023, and connecting to Gare du Nord which departs at 6 pm for London. We would like to leave our luggage (8 total) in lockers and explore for 4 hours, maybe to The Basilica of Sacré-Cœur de Montmartre and the funicular. Would this be feasible? Thanks for any suggestion you can give.
Yes I think it's possible to visit Sacre Coeur from Gare du Nord with 4 hours.
From Gare de Lyon, the RER D to Gare du Nord is quite quick (2 stops, direction Creil). The storage lockers are in the SW corner of the station, one level below street level (look for "Consignes"). There is a change machine on site, but perhaps they've installed a card payment system by now.
From Gare du Nord it's only a 20 minute walk to the base of the funicular so it's quite close by. It's love to walk around the neighbourhood at the top of the hill behind / around Sacré-Coeur and look out over the city.