Decorarate an atruim

How about drapes? You can use layers, sheer colored= so that you don't block out the sun entirely.

Does the atrium get any air circulation?

Use fake flowers, pot them in different sizes in a common theme. Set them around the room in groups of 3 using different heights. also use grass mats and if you can find them, grass shades to fit the windows. You could make a palm tree and grass look, ect....

Rubber mulch is going to make it even hotter in there. It retains a lot of heat. Maybe think about using "grass" carpet.

No one will be going out there so I guess the heat won't make a difference.