Hmmm....it's well known on here that I love Paris Walks. Not a shill for them, just enjoy them. They are 15E, no booking needed, just turn up. Out of maybe 10 or 12 I've had just one mediocre guide. If you have any interest in WWII, I'd suggest the Paris during the Occupation tour that starts at the Tuileries Metro stop on Sunday afternoon. It is excellent. I also really enjoyed one on Jefferson and Franklin in Paris - very interesting!
Have you all watched Midnight in Paris? I'm not a Woody Allen or Owen Wilson fan but I love this movie. The church steps where he sits to get picked up by the time traveling taxi are at Saint-Etienne-du-Mont which is near the Pantheon. The inside of the church is also interesting - relics of Sainte-Genevieve, patron saint of Paris, are supposedly in a box there and there is a neat double spiral rood screen.
One year I tried to find a number of things that are listed on Atlas Obscura including the statue of a lion eating a man's foot at the gate to the Jardin des Plantes. Speaking of which, the Jardin des Plantes is lovely in April!
I also look at Corey Frye's walking tour videos to find interesting stuff. He did a walk one day that included the Church of St Paul and Saint-Louis where he showed a revolutionary slogan left over from 1800's I think that is unable to be removed! I also loved his walk on lesser known sights around Trocadero. Go to Youtube and put in Corey Frye.