I lived Paris for about 10 years, returning to the States in '98.
A lot of the Paris that tourists get to see is pretty nice, so I wouldn’t get too worked up about that. Heck, going up in the Eiffel Tower is fun. Another really standard tourist thing that is still enjoyable is to take one of the Bateaux Mouches (“bug boats”) that ply the Seine. If you are going to be there in August, the place will appear to be populated mainly with tourists anyway, as much of the local citizenry will have decamped for the beaches.
Offbeat possibilities include a trip through the catacombs (access from Place Denfert-Rochereau, open only for a few hours on Tuesdays and Thursdays at last recall); renting a bike and riding out of the city along the Canal de L’Ourcq; the Passages, or covered shopping arcades, that still persist in some parts of the city; a stroll through the Marais section of the city, between Chatelet and Bastille; open-air movies in August, shown in the gardens of the Parc de la Villette (avoid the science museum there, though, it’s a thumping bore). In the evening, check out a couple of the funky bars in the Marais or on Rue Oberkampf.
Here’s a description of the Passages, with links to some of the other sights mentioned above (sorry, it’s in French):
There are not too many areas to downright avoid, but probably the least interesting from the tourist viewpoint are the far northern areas of the city (other side of the Montmartre hill). I cannot stress enough, however that any money and valuables you carry arround with you should be securely hidden away on your person. While violent crime is nearly non-existent, pickpockets are abundant and highly skilled.
Depending on the length of your stay, I strongly recommend getting a “Paris Visite” (three-day) or Carte Orange (weekly) ticket for RATP, the city public transportation system. These tickets can be purchased in any Metro or railway station. Ride the Metro when you need to get somewhere relatively quickly, bus when you want to see the sights.
To keep from looking like a typical American tourist, simply avoid the white T-shirt w/printed slogan-khaki shorts combo that makes Americans look like amusement park refugees.
As for restaurants, there are more than 10,000, and the majority are good, if a bit more expensive than in the States. Some will be closed for the August vacation, however. Personal favorites included La Dame Tartine near the Pompidou Center, the dripping-in-tradition Brasserie Lipp in Ste Germaine, the extremely fancy (and expensive) restaurant in the upstairs of the Gare de Lyon, and a funky little Japanese noodle joint called Higuma, on Rue Ste. Anne.
If any are to be avoided, I would say don’t bother trying to eat in any of the places around Chatelet or the ‘Ptomaine corridor’ around St. Michel.
Harvard Univerity Press’ Let’s Go to paris is full of solid, fairly reliable information on where to go and what to do. If you can read French, the magazine (and web site) “Pariscope” summarize what’s going on around the city each week:
http://www.pariscope.fr/
I thoroughly enjoyed living there and hope your visit goes well.