Below there is a summary of selected extra activities in Rainier, specifically those, which may prove a bit problematic.
1) Mobile Command Center - Quite exceptionally, there are no Jammers in Rainier district, but what may surprise you even more, is the fact that the enemy base is not on dry land. It was raised on islands located in the eastern part of the district. Once you get close to it, you have to be ready to shoot down two enemy helicopters that will take off with one purpose only - kill Delsin. Try to destroy enemy flying machines one after another, at any given time fighting with only one of them and avoiding powerful rocket fire from the helicopters.
Shooting down the two helicopters is not enough, because while trying to approach the mobile command center you will also be disturbed by snipers positioned on smaller islands scattered around the central and largest one. Try to deal with these opponents using ranged attacks, and only after you eliminate most of the snipers, proceed with your plan of reaching the mobile command center. Depending on your preference, either use floating ability to get to one of the smaller platforms or, bounce off from the roof of one of the nearby buildings and glide directly toward the main platform. Of course, try not to fall into the water during the crossing, because once you return to dry land you will have to start the whole process again.
2) Hidden Camera - The camera you seek is in the place shown in the above screenshot.
3) Audio Log - Once you start searching for the audio log, you need to get on the roof of one of the nearby buildings and then jump down to the cornice, as shown in the picture above.