Two pranks I thought of in the past week.
1. Build a set of double doors, which swings open in the center. However, in the center there is actually a pole, but it should be really hard to spot. Have the push plates on the door meet at the center, separated by a thin slit which goes through the entire center pole. This will give the illusion that there's nothing between the doors... and when people try to walk through they'll smack themselves nicely on the pole.
2. Another door trick. Put a piece of glass behind a set of automatic glass doors. People'll do the rest.