It was Tomas, NOT Simon.

A lot of good points being made here. Just wanted to add a couple more thoughts.

*Spoilers*

First of all, we have to keep in mind that Guillermo Del Toro (I know he was only a producer, but still) has stated that he doesn't like the "Hollywood notion" that all "magic" (i.e., supernatural elements) has to have a logical and reasonable explanation. He likes to present these elements as things that are what they are. It is my feeling that this movie carries this same idea. I reference Pan's Labyrinth as the most obvious example to this. While no one other than Ofelia in that movie sees the "magic," Del Toro himself has stated it was real, and honestly by just accepting it as real without logically explaining it actually makes the thing as a whole make sense. Also, it is a common movie theme among magic/ghost stories that - to a certain extent - supernatural entities can physically interact with our world.

All that said, I personally follow the belief that it was Simon who pushed Laura into the bathroom. Now, while I can't prove this, I feel that it better fits the narration of the story.

A couple things to keep in mind. We have NO CLUE as to exactly how long Simon was in the space PRIOR to being trapped inside. Also, that being said, we cannot definitively prove that the game was set up AFTER Simon got trapped inside. A couple reasons for these. One: We see Simon outside of the room within minutes of guests arriving, which means he couldn't have been in there for more than a couple hours. Also, nobody sees him or is even attempting to pay attention during the party, so while it may be a safe assumption to say he was in the basement all that time, we can't prove that. Two: It is a POSSIBILITY (not saying a fact) that the doorknob to the basement was off the door PRIOR to it being closed. We do know that Simon had found the room a while before he even told Laura about it. It's possible that Simon did set up the game, and used the doorknob as the last clue. Knowing that if the door shut all the way without the doorknob he wouldn't be able to get out, he left the door slightly ajar. But while searching for Simon, the poles fall in front of the door, which could have shut it completely and therefore left Simon trapped behind it.

All that - to me - says this: it's a definite possibility that Simon set up the game. It also says to me that it's a definite possibility that it was Simon who pushed Laura in the bathroom. He wanted to show Laura the room, she refused, so it's possible that he set it all up (including instigating her by pushing her into the bathroom) to FORCE her to come look for him and find Tomas's "casita."

On a side note, someone mentioned that Simon had a 180 degree attitude shift in the first game after he found the dosier with his information. If I remember correctly, he doesn't read the dosier when they find it. He finds the object in that drawer, and Laura is alarmed when she realizes it's the drawer that has that information in it. However, though he doesn't read it, he ALREADY KNOWS what's in the dosier and gets mad at her when she reprimands him for getting into a drawer he shouldn't be in. This says to me that he had ALREADY FOUND that document, and potentially set up the game as a way to confront her about it and saying that he already knew.

Keep in mind that the roll of ghosts in many ghost stories is to reveal a truth or reveal something that has been hidden. It is this notion that suggests to me the ghosts in this movie do exist. Going off the suggested notion that those closer to death have a stronger connection to the supernatural, it makes sense that Simon could see ghosts, as he was diagnosed with a terminal disease that Laura was keeping secret from him. The ghosts helped Simon reveal this truth, and I believe attempted to help Simon reveal the truth about Tomas. Then, once Simon got trapped, they attempted to reveal the truth about Tomas's "casita" and Simon's location to Laura. The ultimate irony is that the delay in finding Simon came from the insistence that there was no supernatural element in his disappearance and that Laura was imagining it. If they had abandoned the "logical" pursuit of Simon and let Laura "listen to the ghosts" from the start, I presume they would have found Simon a lot sooner.

The key to immortality is first living a life worth remembering.