Outside there are 2 stair cases that lead to the front entrance of the visitor center. In the back of the visitor center is a ladder to the roof which also has a door that goes to the 2nd floor of the visitor center.
From the front entrance is the front desk with the Hoover Dam Snowglobe on it and a terminal. To the left of the front desk is the elevator to Hoover Dam offices, and in the back is a door to the Hoover Dam storage closet. On the 2nd floor is another elevator to the offices and in the back is a stair case to the roof. There's also another door to the exterior of the Hoover Dam.
On first approach to the exterior main entrance, you will see a generic NCR Ranger standing outside the door. Talking to the ranger will reveal that Colonel Cassandra Moore is a hard driving, tough and scary leader. This adds color to the quests involving Moore, such as For the Republic, Part 2.