Hoover Dam is the second largest in the US, and Lake Mead is the largest man made lake. All the fun facts and trivia provided here were interesting, and the sights are worth stopping by.
I paid $10 to park in the structure, which was just above the food and shopping area. Access to the dam is free otherwise. You can walk across the top of the dam from Nevada into Arizona. There are restrooms there as well. Cars drive by, so be careful crossing the street. They have plenty of crosswalks as they are pedestrian friendly.
I opted for the $30 dam tour because I wanted to see what's inside the concrete-filled dam. Half of the tour was available for the power plant tour. We visited the diversion tunnels and the generators on Nevada side. The remaining part of the tour was exclusively for the dam tour where we walked the actual tunnel that led to the vents and visited the stairway leading from the bottom of the dam to the top. We had a lot of good photo ops, and our tour guides were good. They had a lot of good info. I wouldn't say they were entertaining or fun.
For visitors, I would recommend arriving early because the tours are first come first serve. I got here one hour (10am) after opening on a Sunday in March, but the next available tour wasn't until noon. I had two hours to kill walking up and down the stairs, exiting and entering the security. They have a newer "museum" with an updated video downstairs, but they also have the older audio version outside where the architecture is. You need a ticket to enter, but be warned that you may fall asleep.
Four stars actually goes to the soft serve ice cream cone that only costs $4 at the one-woman cafe. It's deliciously creamy and worth it on a hot day after your tour is over!