La Rive Gauche is where the artists, writers and philosophers are --the heart of bohemianism & counterculture. Stroll around, soak it in! Say hi to John for me!
Probably my favorite museum in Paris, mostly filled with Impressionist art. Make sure you go to the top and walk behind the clock. It's a really nice shot.
La Sorbonne is the most famous university in Paris abroad. It's also where Mai '68 started... Sous les pavés, la plage!
The Catacombes de Paris is an underground ossuary which holds the remains of about six million people. Check their website for opening hours. Lines are LONG.
Out of the way, the BNF is one of the most impressive libraries in the world. Since you like books and architecture, you should swing by.