Montmartre is one of my favourite places in Paris and a MUST do each time I have been.  Montmarte is really the name of a hill on which sits the Sacre Coeur. but just around the corner is a part of town historically where Artsist used to congregate and although now it's a little tourirty - they still do.

There are heaps of bistros and restaurants in which you can just take up a Pew and watch the artist's mill around looking for their new subject to draw.  This is their main source of income so can be quite pushing.  Make sure you view some of their previous sketches or that they are sober enough to draw, in order not to get stung - Like me.

Its a great way to spend an afternoon and either have a bit of fun and get yourself a charicature or try a more formal style of drawing. 
Either way, its a day you'll never forget :)