An awesome site to see! It's worth taking the power plant tour. I think it cost about $15 per person, can't remember exact price, but it was totally worth it. You watch a 10 minute video, the take an elevator down into the part of the dam where the turbines are located. Complete the tour with a walk across the top of the dam. We even saw a couple of big horn sheep on the Arizona side of the dam on the rock cliffs!
Go early by 9:00am or be prepared to wait in traffic entering the dam area. There is a security stop and and the back up can reach well over an hour by 11:00am on a weekend or holiday.