Spent ~3-4 hours in the area.
1) Walked across the new Mike O'Callaghan - Pat Tillman Memorial Bridge which was the 1st turnoff. There are plaques posted along the walkway with pictures of how the bridge was built. It's hard to fathom how much work went into it. But then I look at Hoover Dam and think how amazing it is that they were able to build such a large dam back in the 1930s.
2) Parking Structure $7! However, I would not have wanted to sit in the traffic to cross the Hoover Dam just to get to the Arizona side for free parking. Then again, we sat in traffic to park in the structure.
I enjoyed walking along the road on the Hoover Dam for the views.
3) The Powerplant Tour ($11) included a historic film which was old, but fun to watch, a tour to 2 sections of the powerplant, and the museum. We didn't stay in the museum long as it was really warm inside on a 85 degree day. We opted not to go on the $30 Hoover Dam tour which allows you to go to 2 sections of the actual dam.
4) Train Tunnels- If it were a cooler day, we would have done the train tunnel hike which goes through historic train tunnels to Lake Mead. I saw a sign on the road on the way in between the memorial bridge parking and the hoover dam parking.