Greenwich Market is a fantastic place to be. Lots of interesting stalls selling small press books, clothing and jewelry from local designers, interesting handicrafts and a delicious number of food stalls.
I must admit though this is the stall I always seek out...
Ruby Tuesday's Bakery sells a variety of animal free cakes, biscuits, slices and other delicious deserty things that are "Handmade With Love". Ruby tends to go towards seasonal flavours with spiced pumpkin and chai latte being some of the new cupcake flavours for autumn. There's also triple chocolate, vanilla and strawberry, chocolate and fresh mint, banana choc-chip and mocha to name but a few. Larger cakes are also available by the slice and she also does gluten, wheat and sugar free treats as well. Unlike many cupcake vendors the cakes are always really moist and have subtle flavours that are not over powered with sugar.
My regular favourite is the Lemon and Blueberry cupcakes...
Ruby Tuesday's is at Greenwich Market on Saturday and Sunday as well as being at various other festivals. If you are London-side do check her out for some of the best cakes in town (actually I really mean the world)!