The key in the wall, the numbers on the clock, the number code being at a weird angle and then there being II and FIVE, the toilet, the guys head popping up every once in a while, the floating yellow cube, the tv channel, and of course, the door being unlocked the entire time.
I always try the doors first, because what if it was unlocked the while time, and was so pleasantly surprised. I came back to play again just to see what else there was and was not disappointed!