Definitely go to Shakespeare Books in Paris (
www.shakespeareco.org), it'd be sacrilege not to - Its opposite Notre Dame and its owned (and still run) by George Whitman (a yank) who opened it in 1951 - its a real aladdins cave, it has thousends of rare and obscure books in there that I'd kill to own and its not that expensive. its floor-ceiling books, and has an upper floor that can only be reached by this rickety ladder-cum-staircase, lots of nooks and crannys to hide away and read in, cats walking around (and sleeping) everywhere.....I guarantee you'll love it. It has a great poetry section amongst others...
Notre dame is good, but prepare yourself for a long queue (took about an hour last time I went), and if you're staying anywhere in 'downtown' paris (on the same side of the Seine as the Louvre) then dont bother buying lots of metro tickets - Paris is deceptively small I find (well, compared to London anyway) so you can walk from one side to the other in about 15-20minutes. Also, the best restaurants are found by meandering down random streets, away from the tourist hotspots.....oh, and its cheaper to buy a glass of wine than a glass of coke, so enjoy the boozing when you're there! (although Beer is another matter! Its about 10euros a pint!)
Its worth checking out the Louvre obviously, although personally I find it all a tad dull, but then I have spent my life in and around london so I'm used to decent museums - I much prefer the musee D'orsay over the louvre, but then its no surprise as it has modern/post modern art in there, which is lacking in the louvre (obviously).
Oh and finally, if you see the Mona lisa, tell her she's nothing special from me......