When you are around Churchgate at meal times, a good place to try out is Stadium Restaurant. It is hardly two minutes walk from the station and open throughout the day. You get reasonably good vegetarian as well as no-veg food.
In terms of snacks they are somewhat limited, but if you are feeling just a bit peckish, you can enjoy yourself without being set back by a large amount.
At the same time, it is a bit too open. Being set on the main road, it should have glass doors or something to set it back from the road.