Beggars & Aggressive Vendors, Paris
Watch out for buskers and beggars on the Metro. It seems Paris has no restrictions on this (or at least poorly enforced) and nearly every train I was on had a busker. Some are creative but others just go along the aisle with their hands out. There was one who put on a particularly entertaining show with puppets, so I gave him money but in general you will give out a lot of money really fast if you dont watch out.
The Effiel Tower is nice to look at from a distance and the view from the top is fabulous, once you can wade knee deep through all the hawkers trying to sell cheap copies of tower and other questionable objects. If you have no intention of actually going up the tower, view it from a distance so as not to be bothered by these people. There are regular police patrols that move the hawkers on but not for very long! If you really must go to the base of the tower, you better get used to saying 'no thankyou' a lot (until you get so fed up with it and your language turns slightly more colourful!)
Gypsies. They are very much a part of Paris and their tactics change from year to year. I have been there when all they do is lie on the ground with a basket and a note asking for donations. Some years they are more aggressive.
Do not get off into little side streets alone. You could be accosted by a group of small children with cardboard pieces. They will approach you from all sides and pin your arms down. Then they will grab your purse or camera and what ever else you have of value. It has never happened to me, but I have friends who have been attacked in such a way.
Also, wear your camer and purse straps across your chest. Do not hang them off your shoulder. That makes it easy for some one to walk or run by and grab it from you. Also keep your purse and valuables in front of you with your hand on them.
It's just simple precautions that will make your trip less stressful.
It looks like there are 2 kind of homeless people in Paris; the local ones who know there way around without interfering with the tourist and foreign people who beg for money at busy places.
Around the Place du Tertre and in the busy rue de Chavellier de la Barre there is much change portrait painters will stop you and lure you into having them paint your face.
This one was especially agressive. Far more than his kin, which were 10 times his size. And he eventually wore us down and we gave him some bread crumbs.