How long will you be in Paris for AND is this your first trip? I am all for the approach of just walking around - being a flaneur is great fun, as long as you don't wind up feeling like you've really missed something great one corner away, just because you didn't know it was there. So... My suggestion is a book that I bought a few years ago right near Notre Dame for less than 10 euros. It's called 4 days in Paris, published at Editions Lecompte, and it highlights the main attractions by area.
Also, the Louvre and the Musée d'Orsay are open late on their "nocturne" days, and the admission fee is lower as well. I recommend those especially if the weather's fine for walking around.
One thing that I recomment as well, when you get to Paris, is to pick up a copy of the local (and free) gay 'journal'. One (among many others) is called Agenda Q, (and I believe it's a play on words, as Q is the same as "cul" which is means "ass" but also "sex"; Avenue Q's tagline is "pour les rendez-vous Q"... It is small enough to fit in a back pocket, and is available in practically every bar, restaurant, bookstore in the Marais, and will get you up to date with not only the names and addresses for the bars - all categories, from the hip stand-and-model, to the leather-pits with downstairs playrooms and all-out nudity-mandatory sex-clubs - but also additional information on the various theme nights held in each of them.
Hope this information helps.
