Hoover Dam was pretty cool. Read up on the history before you go and take in how impressive this structure really is. It was like 110 degrees when we went in June.
Overall if you don't do the tour you will see the dam in about 30 mins pretty short so don't expect to see much but the dam, bridge and lake mead.
They have a gift store and snack bar so that was nice. They have elevators for people with wheel chairs.
If you want free parking keep driving on the dam and park on the Arizona side. If you want the closest parking it will be 10 bucks.
Arizona parking has a slight hill so just FYI.