Ideas:
Hire a Velib on a Sunday. They close the Quais along the Seine from 9-5 or so, so you have a traffic-free route (most) of the way through the city. Costs €1 or €2 for a day pass then first 30 mins of each hire is free, then €1 for the next half hour.
Picasso museum is very good (in the Marais) – small so entirely managable (unlike the Louvre which is too vast to see in one visit) but really focused on one artist and his story of Picasso and explains the development of his art and style over time.
Go up l’Arc du Triomphe and watch the city from there, and the madness that is the traffic on l’Etoile.
Go to the Brassiere D’Ile St Denis for a Mutzig (if you’ve been to Morzine you’ll know Mutzig).
The Catacombs are also good (down near Montparnasse) – they moved all the bones out of Paris’s cemetaries in the 1800s and transferred them to disused limestone quarry tunnels under the city. Fascinating, if a little creepy.
Cemetiere Pere Lachaise is also good for a wander, but a bit of a mission on the metro.
Otherwise it’s a great city for a wander. You could probably do that for a weekend without any plan if it’s decent weather. For a wet day Musee D’Orsay is great, more modern work (impressionism onwards) and again quite manageable in an afternoon.