The quickest thing to grow at this time of the year would be a radish.. takes about four or five weeks from sowing to harvest. Just do them in any old pots, maybe get a selection.
The other thing that springs to mind is Chilli peppers, but then you are looking at six months.
Tomatoes are great, if you are growing in a pot, have a look for a type called Maskotka ..they dont get much taller than 15 '' and are suited to container growing where they tumble over the side. They do produce a vast amount of fruit for their size. The biggest bonus it there is no maintenance to them, other than watering. I always do about 30 of these every year.