There are many things to be activated in Der Riese with the X button or Square. I am going to explain where and what they are.
Radios - there are 7 radios scattered in various parts of the map.
We'll start with Teleporter A. In the hallway right before the turn to trench there is a outcoming wall. On top there is a chipped ledge which contains a walkie. Hit X on the walkie to activate a dialogue.
Continuing. Inside the room with the big generator. In teleporter A. There is a furnace that is open on the right side of the generator. Near the stairs and the window.. Inside this furnace there is another radio. Hit X on the radio to activate the dialogue.
Moving on to the mainframe. There is a walkie in the spawn next to the big downward ramp to the window with the burning desk. It is underneath the ledge all the way to the left next to the barrel. Hit X to activate the dialogue.
As we move on into Teleporter B. Upstairs above the thompson, there is a room with a pile of desks in the corner. It's the room after Double Tap. To the left. It is tucked between the two desks next to a pillow. Hit X to activate the dialogue.
Moving on into Teleporter B there are two vats with steam coming out of them. In one there is a body floating legs up (near the hanging man opening/balcony). There is a ladder connected to this vat. On the latter on the lower rungs (easy to miss) there is another walkie. Hit X to activate the dialogue.
Moving on to Teleporter C. Inside teleporter C there are 2 bookshelves/metal shelves. On the top shelf of the shelves near the stairs up to the balcony there is a radio. It half leaning of the edge and is impossible to miss. Hit X to activate the dialogue.
The 7th and final
known radio is at the end of the little underground tunnel in teleporter C. There are 2 ways to reach this radio. There is the hole in the floor to the left after you go up to Maxis' office/Quick revive. There is also the option to go up the stairs that head straight to the teleporter and turn right. From there, go into the room and go way to the end. Then you look behind the piece of wood and there is a walkie half nestled in the dirt. Hit X to activate the dialogue.
Easter eggs.
The first easter egg to activate is in the Thompson room. At the window to the right (you can see the vats) there is a pile of bricks. There is a brick way on the outside of the pile. Hit X on this brick to make it sink into the floor, you find a note.
Corkboards.
When you hit X on the corkboards the characters will say things. I have only gathered two things Richtofen says. When you hit X, Richtofen goes hmm... goot. The other dialogue was "This could come in handy".
If you hit X on unattainable objects the characters will say things.
From what I remember: Richtofen "I require more money."
Dempsey "Damn, I need to kill more freakbags first."
There are 3 Spine/brain tubes throughout the map which are used to activate the song easter egg "Beauty Of Annihilation". The first two are near teleporter A by the stairwell leading up to Jugger-Nog/Bouncing Betties. One is on the desk to the right of the stairs. Hit X to activate it (you will hear a little sound). The next one is almost directly across from that. Look to the left as you come up to the windows and it should be on top of a metal shelf. Hit X to activate (you will hear a little sound). The third and final tube is in teleporter B. Go inside the room, and turn right. There is a little doorway. Inside there is a chalkboard with the grenade chaulk outline on it. Next to that there is a desk with the glowing spine/brain tube. Hit X to activate (the song will start playing a few seconds later). Enjoy your song!
Other
If you hit X while looking at paw prints and other prints your character will grunt.
That is all I have for activatable objects. Thanks for reading!