Rue des Rosiers, Paris.
This is becoming more of a well known Parisian activity, but it is still definitely worth pointing out to anyone who has not yet discovered it - the falafel takeaways in the Marais, just a walk from St Paul metro. There are quite a few to choose from, particularly along this road, and different people will recommend different places. For me, this one is my favourite - just on the corner of rue des Rosiers and rue des Ecouffes. The queue is always one of the shortest, and they have a really nice onion salsa that, if they don't put any on and if you're feeling brave enough, you can ask for. The good thing about there being so many is that the price is very similar at every place, at 5.50 and for this you get a really filling pitta with falafel and salad. For a few euros more, you can upgrade to a chicken filling if you aren't fond of the standard vegetarian option. There is the option to sit in, but I would recommend takeaway and wander around the Marais eating as you go. If you're lucky, you might stumble across a park to sit in, or wander down to the Seine to enjoy a bit of sun.
Keep an eye out for... The free falafel tasters in the bowl on the counter-top.