Below is a summary of selected additional activities in Belltown district.
1) Mobile Command Centre - D.U.P. base is located in the north-eastern part of the district and is surrounded by three jammers. You should destroy them first but you have to watch out because there are many enemies (also with powers) who occupy rooftops of nearby buildings. I recommend using ventilation shafts to get to jammers, so it will be harder to take you down.
Once you destroy all jammers, focus your attention in mobile command centre. It is located on a small yard between buildings so you'll have to use some kind of powerful attack from the air. Watch out for enemies with powers, as well as standard soldiers and automatic guns (it would be best to switch them off - press touchpad once you get behind them).
2) Blast Shards - In Belltown district you can find few hidden drones. One of them is located on the rooftop of a skyscraper. Since the game doesn't allow you to use ventilation shafts to get there, you'll have to climb it up.
Two other drones need more advanced climbing - first one is located on the top of a large totem and second one can be found on the neon adjacent to Super Car Wash.
3) Hidden Camera - Camera you're looking for is located in the place shown on the above screen.
4) Audio Log - You have to get to the rooftop of a nearby building. Audio log is located on the ground, next to huge air conditioner (screen above).