Make certain props such as fences actual props, just frozen.

Make the windows of the mall glass_surf, but make them get deleted if any support next to the glass breaks, to avoid the weird border. (easily done with logic, I could show you)

Scatter some more physics trees around the map.

Set physics trees to asleep, so they only gain physics when disturbed.

Trash objects (takeout, cartons, etc) around populated areas