Originally Posted by ILuvParis
Another option is a crepe stand - which seem to be everywhere, for a savory or sweet crepe. I have a friend who, when travelling alone, will often eat "street food" almost exclusively while in Paris.
I'd give a strong second to the creperies--you can find them everywhere, they're always cheap and have good varieties depending on what you're in the mood for. A word of advice, though, if you're not going to sit and eat, is to wait a minute and let the fillings partially resolidify, otherwise, there's a good chance you'll end up with a trouser leg full of cheese/chocolate/butter/etc...