I acrually visited Jakarta Tua-- the Old City twice with some Indoneisan friends this year. First time was in the early evening when the Plaza was busy with families eating and playing futbal, as well as tourists-- Got a chace to some fotos of the old buildings and of course, the cannon and just enjoyed walking around.
Second time we came in late afternoon- had some light food, and did more walking- taking in some of the nearby Harbor area. The Colonial buildings were great, and we also spent a few minutes in the 'night bazaar' that was setting up on the outside perimiter. Nothing there that I was looking to buy-- but still enjoyable.