We really enjoyed our visit to Pettah Market. It's massive and sprawling so I recommend planning your route first (we didn't do this and I think we missed some of the good stuff because of it).
It's definitely worth visiting the fruit and vegetable section. The stall holders were friendly and very kind, they were happy for us to take photos and sample/ask questions about some of the fruits we had never seen before. Trust me, don't try the sour cherries, they are like "toxic waste" (disgusting kids sweets) but a million times worse. I caused much hilarity when I innocently tasted!