To me, the dam and the bridge are some of the greatest achievements of man made structures over the last 200 years.
The parking is $10.00 for a car and from there you can walk around anywhere outside and take pictures from both sides of the dam or walk the bridge if you want.
Make sure to stop by the bronze statues down by the dam and rub their feet, it will bring you good luck.
If you want to tour inside of the dam you can go down into the visitor center for a tour. If you like things like that the price is well worth it.
Across the dam on the Arizona side you can also park and see some wonderful views. There is paid parking and also a small "free" lot just a little further up the road but it's usually hard to find a spot so I just pay the $10.00.
If you don't have a car, you can find many tours for $39.00 or so which are well worth it and will include picking you up from your hotel. They also typically help point out landmarks along the way and might even stop at a store or two to pickup souvenirs.
Enjoy!